When is it a plateau?

I apologize in advance for yet another plateau thread but my mind is swimming all the information I find when researching this so I am hoping to get some input from folks who have been there done that and maybe can encourage me or give advice.

Background: I have lost around 33-34 pounds since January. Been losing a nice, steady average of a pound per week, with a net of 1200 per day.

Initially I was doing a mix of light cardio and weight training but in the past few months as my weight has dropped and my fitness improved I have found a love of running and have been primarily getting my exercise from brisk walking/running 5-7 days per week - typically 3 longer faster run/walks and the remaining days a 30-45 minute brisk walk.

Even when I dropped the weight training and increased the cardio, the rate of loss remained steady.

Until three weeks ago...suddenly the weight loss came to a complete stand still.

I have been trying to ride it out but am now becoming concerned.

I don't see how 1200 net could be under eating as I am 5 ft 4 and at least 25 pounds into the "overweight" range for my height.

I understand weight loss will slow down as body weight is reduced but I would expect that to look more like gradually losing less each week...this is a sudden stop.

So, is it really a plateau or do I ride it out before trying anything drastic.

I am considering adding the weight training back in but realize that may just prolong the stall on the scale as my body deals with water fluctuations due to muscles being worked/repairing, etc. So, I am hoping to find some way to at least get a small weekly loss before adding anything that will introduce even more water balance variables to the mix.

Sorry so long, just don't know if I am panicking prematurely, if I made a big mistake by shifting to all cardio, or if I am hopelessly stuck.

One more interesting point is that the weight I am currently at is a weight I hovered at naturally for some years about ten years ago. I never really "bought into" the "set point" theory, but this makes me wonder.

ANY help or encouragement, experiences, tips would be greatly appreciated.

thanks!