I had something similar for about a week before I went into labour- one evening it went on so long and was taking my breath away a couple of times so I called the labour ward. They said it could easily be either, to just take it easy and relax - if it is the real thing you will need all your energy soon. It turned out to be pre-labour, but a few days later, when I actually did go into labour if it hadn't have been for my waters breaking I wouldn't have been sure for the first couple of hours. But as they had told me when I called, when it was the real deal it intensified over time and I was in no doubt it was real labour.

Good luck, just relax and enjoy it