she's pregnant

My now ex and I have a 3 month old baby. We weren't together during my last month of my pregnancy and the 1st Month PP. He was with his ex during that time and now she's pregnant. I didn't know how to take it or what to do and I ended it between us. I'm dying inside, even more now that they're back together. The fact that in less than 2 days they got back together hurts even more. The thing is no one even belives it's true except him. I've been EBF the baby and with all this my milk supply has drastically decreased I don't know what to do. I'm trying to be strong and act like I don't care but I do. And I know it was when we weren't together but it still hurts. And the time we were together we had unprotected sex. What if end up both pregnant? OMG I don't know how to take all this. Now he's been ignoring my texts doesn't even ask about the baby, nothing. And I feel horrible about this, but if they do have this baby it'll be their rainbow baby, she had tons of miscarriages before this so at the same time I'm happy for her