masturbation or something else?

Hello. I really hope to find answer to my question. My baby is 19 months old and since she was about 6 months old she was and still is squirming with her legs slowly, like she is swimming or something, whenever she is in high chair, car seat or carriage or any other seat that has back support.  She can do this non stop until i remove her from that seat, and if she does it long enough she gets sweaty. If i'll try to stop her she gets upset and cry. Her doctor checked her for everything and all tests are good, he thinks that she is masturbating. But the weird thing is when she started crawling she started pooping few times every day (before she was always constipated) she stopped squirming, so I thought she was doing it because of constipation, but 1 week passed and she started squirming again, that means it wasn't because of constipation. And it's really frustrating to watch her when she does it.
There are days when she is not doing it but it's really rare.
  I was wondering if anyone ever herd of anything like this before and is it really masturbation or something else? And how long it can last?
Thank you.
I really hope to find my answer here.

[Kevin Kennedy, Ph.D.]

Yes, her pediatrician's thought is likely correct. This sometimes comes as a surprise to parents, but there is nothing to worry about. By the way, re: your thought about constipation (though the behavior is not likely due to that). If a child goes more than a couple or few days without a bowel movement, constipation is likely.