Sounds like your fluid is low, or one of the fluid lines are clogged up. Get your fluid flushed right away and see if that helps. It seems like you would have to have pretty good miles on the car for that to happen, as the trans fluid generally last 100,000 miles. I used to own a 2000 323i too. I loved that car. But one day, while backing out of a restaurant, the tranny quit working in reverse, and I had to replace the entire thing. Replacing it cost me $3,000 on the cheap because I found a used one in a junk yard. I hope you don't have the same problem, but I would get that thing looked at quick. One other thing, the 2000's have two different kinds of automatic transmissions. In the later part of the year, they had the steptronics, like mine was, but in the earlier part of the year was one that didn't allow you to shift gears manually. If you have the earlier one, then you are going to be in better shape I think, as they were more dependable. But the steptronic in 2000 still had kinks. By the way, my tranny went out with 110,000 miles on it. Other areas to watch out for: window regulators can go out, cooling systems problems are common for that car too, like the thermostat needing replacement, and also those little orange pixels on the dash tend to burn out. Hope that helps.
I've worked here for over three years (www.importone.com) and I am a previous owner.