Thanks for the query.
You didn't mention what is your gestational age
With progression of the gestational age, the uterus
becomes large, the baby grows and the presenting part settles in the lower pole.
That can be one cause for your problem.
Low lying placenta may not create pressure in the lower abdomen unless it is completely covering the os etc.
So if the pressure increases once consult gynecologist and get evaluated.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429