It normally only happens right after I empty a full bladder, like at night. Its like a dull achy, crampy feeling for a few seconds in my uterus. My first thought was urinary tract infection so I took a home test and it was negative.  Does anyone else have this? I didnt have this with first pregnancy.                
                
           
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Re: This dull ache after urinating...
*not so expert opinion* Maybe releasing your bladder slightly irritated your uterus? If I recall my biology correctly, the bladder is right next to the uterus, so it's not hard to imagine that releasing your urine may have an effect on the growing uterus, esp. if you're forcing it out. Maybe try relaxing (or relax more) and letting it flow out...see what happens.
Talk to the doctor.