Fainted at the gym yesterday,, Whats wrong with me?

Ok im 28 male, i have been go in to the gym for almost 2 months now, so yesterday i decided to get a upgrade to my programme. The instructor gave me much more heavier weights to do than the previous programme, but near the end of the session i started to feel sick,light headed, lose concentration and before i knew it, i was lying on the floor had a lump on my forehead (must have hit one of the machines as i went down).. I JS don't know what happened. Recently ive been trying to eat alot healthier, more veg and fruit, yesterday morning i had a roll and egg and a glass of promegate juice maybe 30 min before i started the workout.. I dont know whats wrong with me,, i do suffer a bit from low iron and Asthma but not enough to justify what happened. It was embarrassing experience on top of it all...