Strange Belly Button Pain - Have You Ever Felt This?

Okay, so, for the past few years, I've experienced this pain. It's not constant, thank God. It only happens every now and then.

I'll try my best to explain this pain;
It's like a string is attached to the back of your belly button, and the other end is connected to the clitoris. The pain only occurs when the body is extended (standing straight, stretching, etc.), and is quite excruciating.
I only seem to experience this pain around the time of my period, and it's always embarrassing when it happens.

The first time I felt this pain, I was fourteen, and I was at Playgroup with my sister, and her two kids. I had my period at the time, so I was limiting myself to sitting on the couch, watching my niece, who was, at the time, only a baby.
My sister asked me to fetch her other kid, so we could leave. Okay, easy enough. I stood, and almost doubled over. The pain was one of the worst I'd ever felt.
I tried to play it off as back pain, and ended up looking like Quasimoto, trying to avid that pain again.

I'm now eighteen, and I haven't felt this pain since just before my sixteenth birthday. I had completely forgotten about it, until now.
My wishes to sleep have now been destroyed. I cannot move without this damned string pulling.

I'm a very private person, and I haven't told anyone of this pain. My biggest fear is being taken to the doctor, and being told I have to strip down, and lie on a bed. I think I'd rather die.

My question is, has anyone else felt this pain?
Is it just me?

Xx-Johnny3Tears