Help with uncomfortable social situation

Sorry if this is not the best place to ask this. I just want a little help. so there's this girl at work who I've known for a couple of months and we're becoming friends but I kinda like her too. (this part is going to sound like some random romcom bs so just skip if you want: so she missed a couple of days of work cuz she went outta state. during those days I figured how much I like her cuz I missed her. I decided to ask her to hang out, not really a date just to see how things go. she comes back, I go ask her how her vacation went, she says "this good" shows me a picture of her hand with a ring. I don't remember much of the conversation for the next few minutes but I did find out it was just some dude she's been talking to asking her out)

so she just got a boyfriend that I didn't even know she was talking to. that hit me outta left field and made kinda sad which changed my demeanor towards her and now she thinks I'm mad at her. I'm not mad at her at all cuz she didn't do anything wrong but I can't help how I feel. what do I do? tell the truth? let her think I'm mad. Adopt a puppy I find while feeding the homeless, thus demonstrating my value as a philanthrpist nd animal activist; then you know engage her more, nurture our (maybe plutonic) relationship, neglect any negativity, which might inspire some hope in something meaningful, and if not maybe some separation might be best. do I just let her keep thinking that I'm mad which will give us the desired separation. is there a way to tell her the truth without causing guilt or pity. I don't want no pity cuz I'm a big strong man. I'm sorry I'm under some influences and I'm worried it's going to be awkward at work tomorrow. I spend 12hrs there I don't want it to be uncomfortable.