I knew ds1 had a problem from birth, looking back he was born autistic (some people talk about how their kids were 'normal' until 18m/2yr and then regressed. ds1 was different from day 1.
ds2 was a smily happy baby, lots of eyecontact etc and I thought wow, what a difference. In fact, it was the diff between ds1 and ds2 that gave me the courage to push push push. ds1 was diagnosed with autism at 2.5yrs.
ds2 was diagnosed with autism when he was about 2.5!!!
Which goes to show, you really can never tell.
What I would say is trust your instincts. I said there was something with ds1 from birth, hv, gp etc wrote me off. I didn't see anything really with ds2 until he was about 2, then I put it down to copying his brother at first.
If you feel worried, get advice, get input, get noticed. If at the end of the day, they finally come and say we have catagorically ruled out X, Y, Z, you can heave a sign of relief. It cost you nothing and you feel better.
If, on the other hand, at the end of the process, it is discovered that she has something (autism or whatever), then she benefits from early intervention - which is a bloody godsend!!
There really is no downside to getting her seen by someone.