Hi Hazelflower firstly, don't worry!
What you say to them is, everyone reacts in different ways and the fact that you have any reaction at all is good, as it means that their body is responding well to Reflexology. However, any symptoms of pain should be responded with careful words. You don't want to put your clients off or scare them.
Reflexology IMO would never cause pain just after a treatment, unless maybe they have kidney problems to start with. You must have taken a consultation at the start of the treatment, was anything mentioned in there that is connected to this type of pain.
As for being so relaxed that they almost drive the wrong way around the roundabout,(wow you must be good) then maybe you should suggest that they go for a short walk after their treatment before driving home.
Personally, I always give my clients a large glass of water after any treatment and tell them that when they get home, if they should have any reactions like a slight headache or dizziness or whatever it is. Then they should drink lots more water, as their symptoms are just the toxins being released into their system and they need to be flushed out.
However, like lotus flower says, never give tea or coffee as you are just introducing more toxins into the body and undoing all your good work.
Good Luck next time. Hope this helps?