Yes, before you go any further, have the transmission fluid level checked. But it's not a simple thing to do, so you might want to find a mechanic (or someone knowledgeable about these cars) you really trust to do it for you. If you want to try it yourself, here's what a very honest local BMW indy told me to do (which is different from the procedure used when doing a trans fluid drain & fill):
1). Jack up the car at all four points, and make sure it's perfectly level, or maybe even slightly higher on the left, where the fill plug is located.
2) Have a suitable container filled with a quart of your ATF of choice (another VERY contronversial subject) the siphon pump screwed on, the pump primed, nearby and ready for action
3) Loosen the fill plug, and unscrew it until it's just barely keeping the fluid from leaking out
4) Start the car, and then quickly dive under the car, remove the fill plug, stuff the siphon pump's line into the hole, and pump fluid into the transmission as quickly as you can. When it starts leaking out the hole (with any luck, you'll be able to add a fair bit of fluid before this happens), stop pumping, grab the fill plug, and screw it back on as quickly as you can.
5) Let everything cool off for several hours, and repeat the process. If you're able to get a significant amount of additional ATF in the second time, you may want to repeat it a third time.
The way the indy explained it to me that when the transmission is nearly full, which it usually will be, unless there's a leak, fully warming it up by shifting through the gears following the normal method after a drain & fill will quickly cause the fluid to cavitate, and cause the frothy fluid to spill out of the fill hole before the trans is really full. I experienced this first hand. The reason I think this method, strange as it sounds, is correct, is because when I did my drain & fill a year and a half ago, I was puzzled to find that a good half quart less fluid went back in than came out. I assumed I had measured wrong, and called it a day. When I recently followed the method described above, I was able to add a solid 1/2 - 2/3 of a quart, almost exactly matching the previous discrepancy. Unfortunately, I wasn't able to test if topping up the fluid cured the car's transmission slippage when cold, because the very next day, the car was side-swiped by a recycling truck, and has been at the body shop ever since.