Ladies, how would you handle this?

Calle • Just trying to figure out my body

So I started dating my boyfriend a little over a month ago. Everything has been wonderful(of course, honey moon phase) I had not questioned any trust in him and felt very confident in our relationship up until this point. It felt so refreshing especially coming from a previous relationship where I was lied to constantly (about taking drugs, not cheating lies, but still a major issue) after leaving my last relationship I realized how happy I was as a human regardless of being alone I felt so happy, so full and learned so much about what I want and don’t want in my next relationship. Well, I learned this last weekend that my current boyfriend has been snap chatting his last girl every day and he has been deliberately hiding it from me. I only noticed finally because our mutual friend shared what her name was and I saw it flash on his phone while we were at dinner and he quickly closed out of it. I brought it up nicely a few days later and he claimed that it’s because they are best friends (even though he had supposedly been so excited to introduce me to his friends, and he has or had apparently, just weird that this best friend was never mentioned) and also that he wanted to keep their Snapchat streak which was going on 1000 days *gag* who cares we are not in middle school my boyfriend is 25 and this woman is in her thirties with two grown children. Just seems weird. I shared that I was uncomfortable with it and then the subject was dropped but it’s obviously on my mind now and I truly feel uncomfortable and hurt but don’t know how to approach this or fix it, I don’t want to be in another relationship where I constantly feel worried but damn until this point I’ve been falling hard for this man!! I’m upset and don’t know what to do... advice? Thoughts?