That's happened to me but not to that extent. Everytime I sneezed really hard a little pee would come out due to the pressure on the bladder from the baby. One way to fix it is to practice doing your kegals :)
I am similar to ahp84, i just wear a panty liner most of the time. I often find that my panties are moist between bathroom uses. And to to eliminate that odor, I just used a panty liner and remove them while using the bathroom. That seems to have corrected that issue for me.
I actually went to the doctors unsure of weather or not i was leaking fluid, the docs checked and everything was fine, and the panty liners fixed it. It was simple as the comment above, i have pressure on my bladder.
I hope that everything goes well with you. and let us know what the doctor says to you.
I'd go see your OB as soon as possible. If you're not feeling the urge to urinate, but you are leaking fluid to the extent you describe, it sounds like it could be a leak in the amniotic sac, overactive bladder with incontinence, or a bladder or vaginal infection.
I'd say it was normal if you weren't claiming to change your underwear up to four times a day with them being soaked--sometimes when I cough or sneeze or even stand up too fast, I leak just a tiny bit because of the pressure on my bladder, but nothing that would ever warrant a change of underwear.