Some people get dealt a really bad hand in terms of hangovers. I could always drink about a fifth of liquor and do a gram or more of coke when I was in high school, wake up the next morning and feel fine if not good. Others I have known really can't handle drug/alcohol hangovers and get sick either presently or the morning after from drinking 8 beers. Everybody's metabolism and body works differently.
You're taking low-moderate diazepam (Valium) and should have little to nothing to worry about in terms of withdrawal. The diazepam if anything, should be making you feel better due to its long half-life and what sounds like you're relative naivite to the substance.
If I were you guy, I would avoid cocaine as it sounds like it doesn't agree with you. I personally don't like cocaine either, unless it's going into my cubital. Also, use the benzodiazepines in a responsible manner, again as it sounds like you're relatively new to the stuff. Addiction/dependence is possible and withdrawal can rear its ugly head much faster than you would think. Also, make sure you are in control of your acute dosage, you wouldn't want to wake up in handcuffs or the drunk tank now would you?