strong, paint thinner-like smell coming from car

Hello all,

I have a 2010 Nissan Maxima SV. In July of 2013 I was rear-ended and accrued over $4000 worth of damage. Since the damage was supposedly all "fixed," I have been hearing a hissing noise coming underneath my car. Then, I a few months ago, I started smelling this awful smell that smells like paint thinner mixed with heavy exhaust. I am positive it is my car because no matter where I am, when I cold start the car, I can smell the same exact nauseating smell. Sometimes, it gets so bad that it will come in through the vents, even when the car is in motion and will give me a headache. To top it all off, lately I have been going through gas significantly faster than normal. Please keep in mind that this smell ONLY seems to happen on cold starts. Other than that I have not noticed it. The hissing noise happens all of the time when the car is running. My HIGHLY uneducated guess is that when the shop replaced the exhaust pipes, they either did a crappy job (which I have proof of since I had to go back 4 times to this shop to get more and more fixed after the accident and have all receipts showing everything that was "done" to my car), OR something else is screwed up that was not fixed that may be due to the accident. If anyone can offer any assistance, it would be most appreciated since I am starting to get sick of the smell and noise and gas mileage... literally.

Thank you in advance to whomever can offer assistance,

Mark