Can you test right after "implantation" bleeding, or is it too early? Should I wait until the day I was expecting my period or can a test tell now?

In your case,

What you should do is see your doctor asap and tell him/her your situation and your symptoms. They may do the blood test which possibly will take a week for the result. Usually it is best to wait a week after your period is due but irregualar period is hard to judge.

Joe

It is best to wait 1 week after you suspect implantation bleeding, to have a better chance of a correct result, make sure you use the first urine you pass when you wake up in the morning.