Hello,
It could be due to irregular periods or secondary amenorrhea before conceiving and hence the pregnancy is for few weeks only although you missed the periods five months prior. However still I suggest you to get regular ultrasounds done to keep a track of the progress of your pregnancy.
I hope it helps. It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
My best friend had irregular periods since they started. Even on birth control they didn't always come every month. When she started trying to get pregnant, she couldn't. She found out she had a folic acid deficiency which caused her irregularity and to be unable to get pregnant. She now has a beautiful healthy one year old.
Irregularity can be caused by many things, so even though you are in the very beginning of a pregnancy I would suggest seeing a Dr./OB to make sure you aren't at high risk for a miscarriage or your baby isn't high risk for developmental problems. I'm not saying you are, but its a possibility that is best discussed sooner rather than later.