what is this I'm feeling ? 28 weeks today

Any ideas as to what this might be ? I'm going to ask my doctor today at my appt but it isn't for another 6 hours .
This is going to be a weird way to compare it lol but it's all I could think of . 
It feels like if your pants would be to tight and the top is rolling down. 
It only last a few seconds and then it goes away 
But it makes me jump because it catches me by surprise