Was this a reliable source? How tolerant are you to stimulants, in particular amphetamines? Also was the experience any different than it normally is?
If you live in the UK or Ireland there is alot of powder crap being knocked out as speed and MDMA. It is in fact MDPV, which is related to methylphenidate (Ritalin), although around 4 times more potent. It tends to be a little crazier than straight amphetamine and the crash is quite fucking nasty. Just something to keep in mind if you live in those areas.http://en.wikipedia.org/wiki/MDPV
Dizzyness is an effect of a speed comedown as is postural hypotension (dizzynes or fainting upon standing, muscle weakness) and if it was as pure as you say 2-3g is a pretty large amount. As is depression and loss of motivation. This can last a few days, try to eat well, exercise, multivitamins maybe. The reason your so down is that you've depleted chemical messengers in your brain, these take a bit of time to build back up to normal levels. You should be feeling better soon, if not I'd advise seeing a doctor.