Well, like someone said, the only way you will know 100% is ask a vet or get a Weefoal test. (Personally, I learned my lesson.) If you choose a vet, I recommend an Equine specialist, someone who works around them the whole day, not a mixed vet. WeeFoal is very inexpensive way to test. You have two choices. 30-120 day test, then 120-300 day test. If the chance is she is further then 300 days, you don't have long to wait.
Now, on for my opinion. It looks to be a baby bump, but I have proof that mares can be decieving on that part as well. I have a pregnant mare, (Lena) and a non-pregnant mare (Ladybug). Within the last year someone could ask if they both were pregnant. Not because of hay belly. Ladybug is desperate to be pregnant, so she has a real bad case of false pregnancy. Now, in false pregnancy mares can go as far as laying down in false labor. My mare, even after a year of being diagnoised for this and treated for hormone balancing produces more milk then my actual pregnant mare.
So, if you have a haunch that she is pregnant, think wisely about getting a vet out. Although most mares foal successfully, and I get the whole "what did they do in the wild?" thing. Even so, you want to be on the safe side if you love you're mare.
With that, I've ranted and such and got all the do-dads out of the way. Keep us updated on what you find.