Is it possible to overdose on Echinacea?

Echinacea has done wonders for me when I've started to get sick, I usually take 2000mg at the onset(when I start to feel symptoms) once a day till I feel better, then I stop taking it because I was told your body will start to build up an immunity to it and it won't be effective. So I did this, but got much worse once I stopped taking it, so I took a dose of 2700mg and now I'm wondering if it is possible to take too much and if so, what can happen? I don't have any medical insurance, can't afford to pay a DR., can't miss work, and I rarely get sick but when I do, I get VERY sick and will HAVE to get antibiotics to get well again! I'm trying to avoid getting worse, but feel I'm losing the battle. By "sick" I mean I have a nasty cold, fever(low), extremely weak,sinus's achingly full(can't blow it out),coughing up green sputum(small amount), and generally feel awful.