I haven't had my period in two months and I've gained weight and my breasts are swollen but I took a pregnancy test and it came up negative, I'm confused?

Some women don't show up positive on the test till 2nd or third month sometimes, although rare it is possible.

Are you taking the test first thing in the morning as soon as you wake up ?

And to confirm, do a blood test at the doctor.

If you are not, then the doctor needs to discover what the issues are