Is there *any* other way to see if she's pregnant other than an expensive U/S?
If she is pregnant (to be honest, my money is on that, that's alot of weight gain in 15 days for just food) to me she looks past spaying time - she's pretty big, I see why your concerned!
If I were you, I would get the U/S (or any other cheaper way to find out) and start preparing now for puppies (if needed) while you have a little time to purchase things over time so as to not drain your bank. Again, if she is pregnant.
Best of luck! If she is pregnant, how about making a list of things you'll need and post it on chaz? I'm sure we would all be willing to ship a few things (towels, puppy formula, and whatever else you will need).