Sleep regression??

So my LO is almost 15 weeks. She has slept through the night since she was 3 weeks old. Usually 8-10 hours a night. The past few nights she has been fussing, not crying. But waking herself up. I'll try soothing her and putting her back in her bassinet, but a couple hours later it's happening again. And eventually, soothing doesn't even work anymore and it just makes her mad. The only way to calm her at that point, and for me to get any sleep before having to get up at 430 am for work, is to lay her on my chest and we both go to sleep. I really don't want to start co-sleeping. But it seems like that's the only way we both get some good sleep. Does anyone have any tips or suggestions to help??