Advice on guys

Okay so I’ve been talking to this one guy for almost 4 months and we both like each other like a lot. But I have this friend that I’ve flirted with before like we flirt and then stop and then begin again like it’s a on and off thing every year and we started again 2 days ago and I mean I know it’s not bad because me and that guy that I like aren’t dating so in my opinion it isn’t bad but then again every time I talk to the other guy and flirt with him I feel this guilt in my stomach and idk why.. i need help please. Is it bad ?? Me doing this ? Even though I’m single . Is it bad ?