Is this a concern?

[asnakenamed-oly]

Recently, I've noticed that my 3-4 month old corn snake has looked a little thin. I was told by a pet store owner that it's normal. He said as she got longer she would look skinnier. I've also noticed she's acted a little different; slower and a little less active. Plus, just last night, she regurgitated. I know that lots of things can cause a regurge, but my sister also had a corn that regurgitated, and a few days later it passed away. This causes slight paranoia for me. Honestly, this particular pet store owner is not a very trustworthy source, in my opinion. I have troubles believing what he is telling me. Outside opinions are all I would like and would be greatly appreciated, as I am quite concerned about my little Olympia!