Need some help ladies :)

Ok so I'm currently pregnant with my 3rd little monster lol but I have no idea how far along i am. I've narrowed it down though, I'm either 10 weeks or 18 weeks. I don't have an ob appt until June 21st but I'd like to get some kind of idea by trying to find my fundus but even two kids in I still have no clue what exactly I'm looking for. I'm a plus sized woman, got plenty of fluff on me so I can't really just look in the mirror and know. I've palpated my abdomine several times to try and find my uterus but I don't really feel anything hard. A couple finger widths below my belly button though when I push in I feel a dull pain in my uterus like it's a balloon ready to pop. The first time I tried the top edge of the pain was lower than it is now. Am I finding it and it just feels differently for me or what? I'm sooooooooooo confused...