It sounds like the transmission may be slipping, if you still have warranty left on the powertrain I would take it to the dealer. It sounds like a transmission issue.
a lot of times a transmission will slip under heavier loads, but not under light acceleration. I still say from your description it may have a transmission concern. If you have warranty left you need to have the dealer at least drive with you and verify your concern to find out if it's something you can get covered.