I have had both, I take stemetil still for migraine-type symptoms which will be similar to your nausea and I find it very effective.
I was prescribed amytriptyline many moons ago as part of treatment for depression and anxiety, and while it did the trick then, to be honest it was prescribed too long for me and I found it highly addictive. I am sure these days it would be more closely monitored but it took me a very long time to come off them, step by step.
I do not doubt that this prescription for you is for a different and specific reason, and may do the trick - but both are potent drugs. I would not like to make suggestions to you without you realising that I am not a medical person, but if you take Stemetil only as needed (p.r.n.), why don't you start the Ami on the suggested regular basis and see if this reduces your need for an anti-emetic altogether? You might have to live with some nausea while the Ami gets into your system but - if it were me - this might seem a reasonable course of action.
I guess really I would prefer to leave it until Monday, and make an urgent phone call to your GP for their advice, so that the interaction can be noted on your records for future reference. To me, that would be the safest course of action, given the nature of the two drugs involved. I hope this helps - but it is my personal opinion based on my experience of both.