If you are feeling nauseous after a ride, it may be that you ate too much before your ride. Before your ride, eat a light meal and then wait for 30 - 45 minutes prior to riding if possible. During digestion, more of your blood supply is diverted to your digestive tract for its task at hand so there is less going to your muscles for cycling. 30 - 45 minutes sort of evens it out.
If you are already eating modestly prior to your ride, you should eat something with carbs during your ride, something like a banana maybe? Or a power bar or Gu gel or whatever you like that has carbs in it. I prefer fruit because there are no wrappers that need to be carried home to be disposed of, and things like apple cores are biodegradable so you can pretty much throw it into the bushes. Insects and small furry animals love when you do that. It may be that when you are "upping your speed", you are getting close to bonking which is a slang term for carbohydrate depletion. Nausea is a symptom that you are nearing a bonk.
If you feel awful after a ride, it is probably because you are not recovering properly. I suppose that you hydrate while you are riding. Continue hydrating after your ride for awhile with a drink that contains carbs. You want to replace some of the carbs that your body used for fuel while you were riding. Also you need to consume some protein. You have just finished exercising your muscles which causes minor muscular damage and your body needs the protein to repair the damage and revamp your muscle structures to handle the increased stress that you are putting on them.
By the way, if you are trying to increase your speed, the best and easiest way of doing this is by riding intervals. Search the Cycling Training forum for interval training and you will easily find out how to do it. Intervals are not a lot of fun, but they are the best way of increasing your speed without ending the day feeling awful.
