C. - If you've ruled out any illness, and what you mean is that your belly seems suddenly huge (my "expansion" was always right around 5 months, too) then what you're feeling is round ligament pain. My chiropractor explained it to me - your uterus is like a balloon sitting in your pelvis. It can get twisted and sit askew, which causes pain in the ligaments holding it. Now, not all chiropractors know about pregnancy, but if you want relief, find a chiro who does, and he/she can help ease your pain tremendously. Mine gently rubbed in circles on the area of trouble and had me raise my knee up above my belly at a certain angle, and it helped so much! (Not to mention the relief he gave me from the back pain and sciatica that came in the end months! And yet I love being pregnant and can't wait until the right time for #4!)
I would look for a good family chiropractor - it is so helpful for baby, too right after delivery, because their spine and neck often get put out of place during delivery and that makes for a fussy baby. Gentle massage of the spine with the thumbs is all it takes for a chiro to get baby in perfect working order again!
Blessings!