One of the biggest differences between cofounder and employee is the direction of money flow. If you're putting money in and working, you're a cofounder.
Yes, this is certainly true, and I was putting money in as well, but the amount I could afford to. Remember that these founders are worth hundreds of millions of dollars, while I just graduated last December. The best way I could contribute to them was with dedication, effort and work. I wasn't asking for a salary (I got enough to sustain myself for a couple years), just equity, which is another way of putting money in. I was putting in all the money I could afford to, if it went bust, I would need a job after a couple years.
It sounds to me like you were an employee.