Fast calling in online poker can be a tell. But you have to take a note on the players which it can be somewhat reliable.
For me when opponents fast call it can mean most of the times one of these:
1 - drawing hand such as FD or OESD (or sometimes a gutshot depending on how much of a calling station the opponent is);
2 - weak made hand such as TPLK or 2nd pair + weak draw;
3 - and sometimes a set.
What I mean is if he though he had a better hand than yours he would stop to consider raising (I mean probably, because if he is very passive he could just call down).
It can mean something else. It all depends on the villain. You have to take a note on that player when you go to showdown to even think of considering it a tell. And don't ever take it for granted. Also if you multitable you won't be as much aware of those.
In your situation I would try to get to showdown without investing more and definetly fold to aggression. If you could go to showdown you could then make a read on the player for future situations.
EDIT: btw NightFrostaSS, is that your real post count? leet post ♠_evil: