Stomach cramps 28 weeks...

So im 28 weeks today and this morning I've been having bad nausea, my stomach is cramping, I have a low dull backache, and pelvic pressure. I'm not sure how to tell if I have contractions this is my first baby. My stomach and back are really bothering me, but my doctor's nurse usually says im fine when I call the office. What should I do, and what do contractions feel like?