If you're battery is draining after only a couple of days by that much, you have 1 of 2 issues
Either your battery plates are dying
or
You have something drawing on your electrical system when turned off..
Look at all your aftermarket stuff (stereo system, alarm, etc). Meter the voltage and amperage draw.
If you're not finding anything out of the ordinary on those components, disconnect them completely and check draw at the battery. The start checking everyything else.
If all that checks out, it's probably your battery and a battery tester MAY or MAY NOT tell you it's bad. Even if those things say it's good, with all the process of elimination you just did, it's most likely bad..