Washing machine drain pipe overflowed. Worked fine for two weeks then happened again. Any help or advice? (Pic included)

Pic of setup. You can see all the lines connecting to the drain pipe. One comes from the shower (including other things as well? Not sure), the dirty one is from my air exchanger (gross-looking, I know). These both seem to connect behind the wall to the orange-capped pipe for the washer drain: http://imgur.com/a/LY6vP8U

Hi all,

My washing machine drain pipe has overflowed twice in the past couple of weeks. Before I call someone in, I'm curious if anyone here has any thoughts. I'm a complete noob with any handyman tasks. This might be a bit long but I understand more information is usually better. I'm putting a TLDR at the bottom but please allow me to explain as best as I can, in case the details matter:

A couple weeks back, the washing machine drain pipe began overflowing during a load when I wasn't home. My girlfriend called me up and I raced home to try to do something about it. I shut off the water, learned on the spot how to disconnect everything, towel'd up the mess and got a fan blowing on the area ASAP because the water was sitting on the baseboards for a bit before I got home. As soon as I got home, I could see water sitting close to the tip of the drain pipe. When I checked back a few minutes later, the drain had cleared (as far as I could see).

To test a potential blockage, my girlfriend and I took turns pouring large jugs of water down the drain to see if it would back up... Nothing happened - the water was flowing and no backing up. I also tried snaking the washing machine drain pipe with a metal snake. I hit what felt like a hard point that required force to pass, and felt uncomfortable forcing anything out of fear I'd damage something. If you look at the picture I uploaded, there's a clear drain line that comes from the shower (possibly other drains upstairs as well? I don't know...) and connects to the same pipe as my washer drain. We decided to pour some drain cleaning product down the shower in case it would clear up anything where the drains all meet.

We decided to try to set up the washing machine again, since the drain seemed to have cleared and was flowing fine as far as we could tell. After being forced to learn how to install a washer/ dryer (the only good thing that came out of this situation), we decided to let it rip and watch what happens. We sat watching it and chatting for multiple cycles. No back-up, nothing sounded or looked off. We called a plumber and asked if it was still wise to have it looked at and they said it sounded like a blockage may have cleared and to save my money for now. We used the machine many times in the following two weeks with no issues.

Now fast-forward two weeks to last night. Friday night, finally time to settle down after a rough work week. Throw a load in the washer, sit down to relax. I'm sitting with headphones on and suddenly hear the sound of rushing water. Water is absolutely pouring out of the drain pipe. I immediately jump up along with my girlfriend and we shut everything down, pull out the machines, clean the area, throw a fan on, etc etc (much more efficiently than last time due to our new-found experience)...

Now I'm left wondering wtf is going on. Does this sound like a potential blockage, or something more complicated? I find it strange that the first time it happened, the water was sitting at the top of the pipe, indicating, to me, some sort of blockage, only to clear itself and be fine very shortly after. Did the drain cleaner temporarily dislodge something enough to give it a couple weeks of proper functioning? I showered right before using the washer - could this have accumulated enough water to the point where the washer was too much? It wasn't an issue for the past couple weeks so I have to wonder. Maybe my drain lines are positioned improperly? My washer drain line was pretty deep down the pipe - could that be an issue? I want to call someone in to take care of this because I'm so lost with this stuff and afraid to break things, but of course it has to happen on a Friday night and I'm sure all the plumbers near me are charging a lot on weekends (will look into it).

TL;DR: washing machine drain overflowed a couple weeks back. After a useless snake attempt and some drain cleaning product, things seemed to be working fine. Two weeks of functioning like normal only to happen again, seemingly out of the blue, last night.

So what do you all think? Simple blockage? Something else? Any advice or thoughts would be greatly appreciated.

Thank you and have a great weekend.