OMG! My heart is torn between two.

So I am currently in a relationship with a guy I’ve been with. Lil over a year and we are pretty serious. We live together and the whole nine yards. Recently I ran into my ex, who has a gf and a son. And we didn’t talk much, but I got butterflies seeing him. And I’m seriously questioning my relationship. I am happy and do love my s/o, but these feelings I’m having for this second guy is making me question some things. I don’t want to mess up his family, more leave my s/o but I don’t like feeling like this. Any advice? Has anyone else been through this? What should I do?