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  A while back, I was in somewhat similar situation as you with Zoey.  But than I noticed it wasn't her abdominal area that I was touching when she yelped.  It was more on the sides.  This continued throughout few days on/off.  I took her to the vet to get it checked out.  
 The vet checked her over and said one of her knee popped out but she put it back.  With these little ones, it's a common issue.  Most of the time, it pops back on its own which could be why the yelping was on/off.
 
 After that, she didn't yelp anymore...
 
 But just to be safe, I'd say take her to the vet.  Better safe than sorry...
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