If you are heads up, the number of raises can be uncapped (depends on if the house rules allow it or not). I have only seen this live. but a quick google suggests that this is also true on Merge. So, if this was three-handed, you would have be stopped at the normal number of raises. But, two-handed, you could each raise as long as the other person was willing. This is actually standard according to the "rules," but I've never seen it online. I, admittedly, don't play limit online very often.
From Robert's Rules of Poker
3. In limit poker, for a pot involving three or more players who are not all-in, there is a maximum of a bet and three raises allowed.
4. Unlimited raising for money games is allowed in heads-up play. This applies any time the action becomes heads-up before the raising has been capped. Once the raising is capped on a betting round, it cannot be uncapped by a subsequent fold that leaves two players heads-up. For tournament play, the three raise maximum for limit poker applies when heads-up as well.
Edit: In case I wasn't clear, you were playing two-handed on the flop. Once you were the only two players, there was no limit on the number of raises that could be made. Although, each raise should have still been limited to $0.02 more than the previous one. Congratulations. This is probably the best spot to be in when you discovered this.