Friends hanging out without me

My friend group is about 5 people (me included). They’re amazing and I love hanging out with them. They were already friends for about a year and a half before I started hanging out with them more often. We’ve been close friends for 3+ years. I usually get invited for outings like grabbing food, playing games outside, or just hanging out at their house.
This summer I’ve been working a lot for savings and I’m busy 4/7 days a week. I usually work closing, so when they get out of their day jobs, I’m not available. However, they usually do not hang out after work and I haven’t had to worry.
Recently, they’ve been less active in the group chat (I know there’s another one) and when I try to make conversations it’s very quick. The other day was one of my friend in the group’s birthday and there were no plans made. I assumed it was just a family thing. Turns out they all went to Maine to my other friend’s vacation house.
I just feel lonely as I haven’t hung out with them consistently for a long time but they have. I thought since we got closer, I would be thought of to be invited. I’m sure they have a good reason, but it hurts when they don’t talk with me. What should I do?
(Pardon any grammar or spelling mistakes, I typed fast)

Any advice is greatly appreciated!!