Does sleeping in too long make you even more tired? Like, normally I
get not much sleep (maybe around 6 hours? On work days :)) but on
weekends, I get like 12 hours of sleep. I usually feel even more
groggy when I oversleep; is this how I'm supposed to feel? Do you in
fact get more tired when you sleep in too long?

Please enlighten me :)

Yes you do get tierd if you have overslept.

Just as important as a healthy diet is the right amount of sleep. It
seems obvious but many people don't get enough sleep at night. Without
rest, the body and mind become irritable, fatigued, and they drag
through the day. The same effects can result from oversleeping. Make
sure you get the sleep you need each night.

http://www.motivation123.com/zsmarticle9.html

Absolutely.  As someone who has had a variety of sleeping habits (from
hardly any sleep to way too much, up to 12 hours) over the years, I
can tell you that getting 10 hours of sleep is just as bad as getting
only 6.

But don't take my word, because I could just be nuts.  A simple google
search will give you plenty of sources:
://www.google.com/search?hl=en&lr=&q=too+much+sleep+site%3A.gov