Oh this hurts. For both sides of the story. I'm sure your server never meant to make you feel like a criminal. At any given point in time, a server may be thinking "table 10 needs ranch, table 12 needs refills, check on table 14 to see if their food is okay. oh! table 12's food is ready, let me get their food out. Table 14 wants dessert. Table 10 is finished with their meal. Wait, they want to split the check three different ways.Table 14 needs to be cleared and a check ready for them....yadda yadda." And this can all happen in a matter of 15 minutes. Yes, those 15 minutes that kept you waiting while she more than likely went to a manager to get help but the manager probably was busy helping another server.
But on the other hand, you didn't know the process. How could you know if it was your first time? As a server, I'll even tell you that I agree that it is upsetting to be immediately confused because you thought you paid and then felt like you were being stared at by other guests. But also as a server, I can tell you how frustrating it is to make sure your keeping track of your tables while trying to help solve a problem (when it should be the manager doing it). I don't know the server's side so I won't say much more. But! That $20 something tip you left her meant your total was around $100. Your $2 tip you ended up giving her is most likely split between the bussers and the bartenders. At my restaurant, 1% goes to bar and 2% goes to bussers. On a $100 check... you need at least $3 to not "owe" anything. You simply didn't leave her a tip at all. With the "tip" you gave, you made her pay money from another table's tip to cover the percent you gave.
Just some insight.