For me there was a real difference between the discomfort and the contractions - but yes, at the very start it can be a bit hard to tell, especially if your waters haven't broken.
Mine started like period back ache. It wasn't actually painful, just a bit weird and different from Braxton Hicks. I didn't think anything of it the first few times, and then it struck me that I was getting this back ache for a minute every ten minutes or so and that they might just be contractions...
It is quite hard to explain where you feel them because they are inside, and not an area you refer to as much. But I would say basically they are regular tightenings, not that painful, that start to become very regular and a little more painful each time.
But trust me, bar the first few while you work out what is going on, you'll know they are contractions
They really do feel very different from anything else. And hopefully you'll have had your waters break or a show (where the mucus plug falls out, quite gross, can happen a few days before anything starts to happen) beforehand to give you an indication that you might start labour soon.
As for calling your midwife, the general recommendation is as long as you feel ok and the baby is still moving then there is no need to call the midwife until the contractions are incredibly regular or it's getting too painful. Or start a thread on MN and ask people for advice, that's what I did!