I began gaining weight and I put it off as my metabolism slowing down, which my doctor had told me may happen. I have irregular periods so going months without one wasn't alarming for me at all. Once my stomach began to poke out more than I felt comfortable with I decided to take a home pregnancy test(twice) and it came up positive. I then went to theclinic to confirm the pregnancy but I could not tell the nurse when my last menstrual cycle was. Now I think that I am around 4 or 5 months because my belly has grown rapidly and big. How can I find out how far along I am?

You'll most likely be told approximately how far along you are when you have your first ultrasound. Have you had your first prenatal appointment? You doctor may have some idea as well based on your measurements.

Congrats and good luck.