it can be a sign of pre-eclampsia which can be dangerous to mother which is why they will keep an eye on it.
Of course it may have been high because you werent rested and had rushed to clinic or because you were nervy at the clinic. Mine was always slightly high.
A tip i was given by another pregnant woman who had gone through similar was that grapefruit juice can lower BP, although im not sure how true this is.
Go back in 2 weeks, its obviously not high enough for them to be concerned at the moment, they are just keeping tabs on it.