My girlfriends ex messaging her

I've been with my girlfriend for over a year and we're discussing moving in together. She's told me in the past that her ex messaged her and that she told him that she's happy in a relationship and that he should back off.

She recently told me that he messaged her again and she chose not to tell me about this for a few months. She only told me because I was telling her about an unfaithful friend.

I was obviously angry/upset that she never told me. She blocked his number and told me that she's never once replied to him. The strange thing is though that I immediately got over it but she then got really upset and has been distant for a day now.

What else is going on?