Should I be upset?

Lizzy

Me and my boyfriend have been together for a year and 9 months, I recently checked his phone & realized he added a girl on Snapchat who he had a thing for but claims they were never anything, the messages show him sending her long paragraphs saying he loves her etc. her replies are with hearts & all, not sure if I should be upset over something like this? & idk how long its been but it was before we got together. Any advice