In our multiples birth prep class this was the first thing the instructor covered. Her advice, which I found very, very helpful, was to find a soft spot on your belly. At 28 weeks it's pretty much all baby, so it always feel hard. But somewhere you can find a soft spot still - for me it was on the left side, right under my ribs and sometimes on the right right above my hipbone. Then if you're not sure if it's a contraction or a baby moving or whatever, poke that spot. If it's hard, then it's a contraction, if it's not hard, it's not a contraction.
My twins were my first, too, so I didn't have anything to compare it to. No matter what, call your doctor. The worst thing about the multiples class I took was the number of drop outs. The nurse who was teaching it would give us little updates so we knew that it was because they were either on bedrest or had delivered early. I ended up being one of those and my girls were across the hall from one of the other drop outs boys in the transition nursery. So take it seriously, call your doctor!