Why so much interest?

I currently own a credit card and been paying more then the minimum payment each month. Every now and then I notice “INTEREST CHARGE/PURCHASES” on my statement. Am I being charged interest on a specific item I am buying? It’s an inconsistent charge just appears randomly. If I am making payments each month why am I being charged interest?

Probably a stupid question but just lately been paying attention to my statements. I have owned this credit card for 5 years