Has this ever happened to anyone?

It happened last month for 3 days and this month for 2 days. Yesterday was the last day that I felt it and it's gone today. It was also when Glow says that I was fertile, if that matters.

In my right underarm, I would say that it's a tender feeling. I don't always feel it, but I guess at certain times if I move a certain way I'll feel it. It doesn't hurt and I would never say owe, but it doesn't feel like my other normal underarm.. 

I've felt the area and I don't feel anything. But I do have something there that looks like a little pimple, but it's not a pimple. It's been there forever.  

Any ideas what this could be? Anyone ever get this?