Wetsuit = shoulder soreness?

Odds are you didn't pull it up enough. You should have wrinkles in your shoulders when you first get into the water.

You might want to see if they'll accept it as an exchange for a larger size as well, just in case.

There is a slight form change of swimming with a wetsuit as well - a little shorter reach with the arms. If you were trying to do your regular freestyle - that could account for part of the issue.

Could be how you put it on or how it fits you. If it doesn't fit really well you'll be using some of your stroke to stretch the wetsuit each time. Not fun.

I'll assume that you have the correct size suit.  First off - swimming in a wetsuit will add some extra strain to the shoulders since you're now restricting them.  Not a lot, but enough to notice.

Second, as to Daniel's post above, you likely need to hike them up a bit more.  When I started swimming in mine I had the cuffs near my wrists, where I would expect  them to be, but I didn't take into consideration that I have pretty long arms.  So I started to hike them up more allowing more range in the shoulders - problem fixed.

Look at my avatar.  It's not as dramatic now, but this was shortly after suffering sore shoulders and I was hiking the sleeves to the extreme.