There are chances that you could be pregnant.
If your doubts are so high, you should try the urine pregnancy test. They come positive after 10 days of fertilization. If they are positive then it definitely indicates you are pregnant. But, you should know as you are having PCOS, irregular periods are very common. And obesity or gaining weight is one of the symptoms of PCOS. If you have taken treatment for PCOS, that is oral contraceptives for a year or two then I think you should be ready to become fertile. Infertility is a common side effect of PCOS.
If your urine pregnancy test is positive then I suggest you visit gynecologists who will advice you on USG. You need to attend the ANC centers for advices on steps need to be taken to make sure you have a safe pregnancy. You will know which trimester you are in after taking the USG, ideally they calculate by knowing the last missed periods but as your periods are irregular it is difficult to tell when you had the last missed period.