I have an iud and i don't know my last period and i have gained weight and get a butterfly feeling in my tummy .. could i possibly be pregnant? and if so how would i find out ?

Can do a preg. test: A urine pregnancy test done correctly gives a reliable result 3 weeks after sex (3 weeks after when conception might have happened). If done sooner, a positive result is likely real, but a negative result may be due to testing too soon, and therefore lead to a repeat test. If a woman has her normal regular period, then she is not pregnant, and a test is not needed.