Can dogs masterbate?

Question
Hello, I know this seems awefully strange, as I went through a large amount of previous posts and I have not found any other forum post that relates to my Isabelle.  I have noticed in the  past month that when she cleans her self down there, she is getting alot more aggresive in the licking department, then her hips start thrusting.  I was a little in shock and so where my guests.  Then the other night I was wearing my fuzzy slippers, she mounted my cross legged foot.  She is 20months old, female spayed Jack X Chiuaua.
I understand that this is a behaviour thing, but why is she licking to the point that her hips are thrusting?
I am bringing her in for her check up and going to check for infection next week.  I want her to keep herself clean down there but if I say no when she gets carried away, won't that deter her from keeping clean?

Answer
Usually explaining why a dog does anything is speculative.  So it is hard to say if she has gone from just cleaning herself to sexually stimulating herself.  Now a friend, not given to wild tales, was telling me about a male that humped something to the point of ejaculation.  See what the vet finds if anything and if it changes anything.  

While you may be able to reduce it when you are around, I doubt if correcting her will lead to her not keeping herself clean.