feel absolutely devistated

so me and this guy have been on and off together since October due to stresses from uni etc but during our off times we've still basically been together, no-one seeing anyone else or even talking to anyone else. I had to interrupt my uni course until September due to illness and that cause some stress between us meaning were techniquely off atm, yet we FaceTime every night without fail and we both know that when I get back to uni once im better it should be okay. BUT he went out last night for his birthday and madeout with some girl all night and I found out from a friends snapchat. Is it okay to feel so devastated? it's so unlike him to do that, he's so shy normally. I just don't know if I'm being petty by being so devastated and jealous of her?