4 months postpartum and I think my husband is cheating.

I gave birth to our 3rd child 4 months ago. I’ve been feeling disconnected from my husband since the last trimester of my pregnancy. I feel like my husband has checked out of this relationship. Every time I look up he’s glued to his phone. He used to be a super involved husband/father now I have to remind him to get off of his phone. I understand that we’re in a very stressful season of life but he begged and pleaded for a 3rd child. We used to be best friends but now we barely talk. On top of always being in his phone he also started going to the gym. I’m all for self care but I feel like he’s looking for an excuse to not come home after work. Ive secretly gone through his phone and there’s no sign of him talking to anyone. I’m not sure if he’s running away from the stress at home or cheating on me. We used to be so open with each other. I’m trying my best to hold down the fort but I also need support. The first year after birth is always rough but things are 10x worse without having his support.