If it has been sitting, the grease in the bearings might have gotten goopy? Turn it upside down, make sure the wheels and pedals can spin freely.
If the you're low on air, that might do it, too. The tyres should be inflated enough to have no "give" when you grip them.
Road bikes (which I think your Giant would be) also have pretty tall gears: make sure you downshift as low as you can for hills and starts. If you're stuck in high gear, that'd sure give you a ton of resistance.
Finally, and I mean no offense, but if it's only been a few weeks, it's probably just you. When I first started out it wasn't the hipster women who bothered me; it was the old geezers who'd blow past me on the way up hills. Even if you're in okay shape, cycling has its own pattern of muscle use you have to build yourself up for.
EDIT: that's in reverse order of likelyhood, IMO. Sorry.