yes Christine this is all too common - or can be - if too much is taken. The BP will fall and you can crash - this is one of the main reasons they take a sitting then standing BP. I'd say they would have given you some saline quick into your system. If they felt it was that serious otherwise what they do in our unit is make a cup of tea or coffee or some other drink to up the fluids a bit.
And yes, they're right that most likely too much fluid had been taken off - this (crashing) is a common symptom of having too much fluid taken off and it totally sucks.
I can only advise you to keep track of your dry weight yourself(if you don't already) and confirm with yourself how much you should be taking off (as in your pre-dialysis weight take off your dry weight plus a washback - usually 0.5) and make sure they've set the machine to the right amount. Every so often I catch mine out setting it wrong, though these days I usually come in and tell them how much and we confirm it amongst outselves... mostly there is no problem
I'm sorry you had that experience - it sounds like a rather bad crash. You may have also put on some body weight and need your dry weight raised - specially if you notice this happening again....