screaming baby help!

So my 4 1/2 month old gets cranky every now and then however when she gets tired she really fights her sleep (so I thought). She always been cranky so I figured it was normal and I was getting used to it. However when my sister kept her for an hour or two yesterday she was talking about how I should get her checked out. She said that she screamed non stop for an hour almost. We changed her formula just recently to the gas ease one. So her screaming has calmed down a little but sometimes she screams like she is in pain. When you hold her she will start crying and squirming. But when you stand up and slowly rocker her that way it calms her down a little bit. As soon as you sit down she's back at it. Should I take her to see the dr and see what's wrong or is this something that's normal? I'm a ftm so don't know what to really expect.