When the ex gf contacts you!

Last night my child's father ex gf who I've been off and on with for the past 3 years contacted me. She said that they had been hanging out and that she felt she needed to let me know bc things were not adding up with him. She said she was even with him the same night I was with him. A little back information; I caught my baby daddy and this woman while I was pregnant but after our daughter who is 6 months was born, he acted like they were never together again. But she said last night they were even messing around years ago. And she visited him while he was out of town. Part of me believes her, part of me doesn't. I don't know what to do. I ask her how she got my number and she said that she googled it?! Could be lying so they can be together and I look like the bat shit crazy one? She's an older lady (46) so it seems she wouldn't be so immature. But also the kicker is, my baby daddy has not one time contacted me explaining himself. He knows that I know. He said she was lying but I haven't heard another word from him.