Feeling sick can be caused by a number of factors and it's not uncommon for people to look a little green at the end of their early lessons. Trial lessons are often half the length of normal ones so that may have been a factor for you. It's also quite windy today which can make progress somewhat bumpy. I wouldn't be too concerned just yet. Enjoy your flying and don't be afraid to tell your instructor if you feel poorly again. It's in no-one's interests to carry on a lesson with you feeling sick - you won't learn anything - and it may be that your instructor will do some exercises with you that are a little less stressful.
By the way, a sip or two of cold water prior to flying can work wonders.