My relationship feels off and I feel insecure

My girlfriend's grandma passed away 5 months ago. They were very close. She took care of her till she died. Things kinda returned to normal few weeks later. Last month it feels like we hit a brick wall. Intimacy fell off. I asked what's up. She says she can't connect with anyone and that it's not me. She used to be very open and expressive. Now she gives short answers and has no interest in sex or any touching. When we did have sex in the last month, something felt really off. Now I'm very insecure about us and have thoughts of her cheating. She says otherwise, but I don't know. It just feels like something is really off.