<human>: Can you provide a brief overview of the text?

Around 11:00 pm last night a random Instagram account with no followers except my girlfriend [18F], messaged her saying that I [18M] was cheating on her and that she doesn't deserve it. I have been with her for 5 months and I have never been happier, I honestly don't think I would have survived freshmen year without her. 

I messaged this person asking why they were doing this and why they were creepily messaging my girlfriend. They responded that she will eventually figure out what I have been doing and that she deserves someone better like me [Anonymous sender]. My girlfriend is freaking out about this and I just want to calm her down. The anonymous Instagram account was deleted and I have no other contact with this snake. 

My question is, is this something I should work out with my girlfriend and talk to her about possible ex's or odd strangers that she might have seen around lately? or is this something that I should get involved with the University and the authorities?
<bot>: Stranger Instagram messages girlfriend [18F] that I [18M] am cheating on her. I respond asking why and he responds that she deserves better like him/her [stranger].