What Causes My Belly Button Become Harder Suddenly?

I've been workout 6 day's a week and all of a sudden my belly button has been getting harder, the last couple of days. My husband thinks it's a Herina. It doesn't hurt to touch around it unless you push into it, but that's seem's normal to me, cuz if anyone pushing in some where I think that would hurt right? It's doesn't have any Infection. so what do you think? Stacey Winter