What Causes Bump In Pelvic Area With Late Periods?

Hi!
I am a week late on my period, which is rather strange for me. I was VERY stressed two weeks ago, but I also had brown discharge at that time. I am sexually active, but it's always protected sex, and just to double check, I took a HPT, which came out negative. Also, I have felt a medium sized bump in my pelvic area. It is around where I know the lymph nodes are so I suspect it may be that? If I touch it too much, it begins to hurt. In fact, it didn't even hurt until I found it!
I am going to the doctor soon, but I just wanted to get a general opinion of what could be going on?
Thank you!