Teething could definately put her off her food. A tooth gell to numb the gums worked wonders for us (we gave it to ds before meals and he could eat ok, but would not eat without the gell because of the pain). We used bonjella, but there are others on the market. Also, we found that if the bonjella helped we knew ds was teething. If the bonjella did not help, then we knew it was something else other than teething. Having said that, it is a bit unusual for teething babies to sleep so well (you might just have the exception!)
Persoally, I wouldn't worry about the weight loss. Kids do tend to slow down at times. Also, she is still gaining weight (even if much slower). Remember 50centile is the average, so at 75 your dd is still above the average weight for her age. Anyway, those weight charts are only an average and many kids fall above or below them. (I had a friend whose baby was on the 2nd centile and even dropped off the bottom end of the chart - but doctors said not to worry, as she was otherwise happy and healthy and developing well.)
The real issue imo, is whether your dd is well otherwise. If she is alert, happy, and generally herself - then she sounds fine. If she is floppy, listless, off-colour - then take her to the doctor just to be sure (they'd rather have you pop in even if it is nothing, than miss something that might be wrong).
Have you tried other types of food (she may be bored if you haven't introduced new foods recently). Or is there an old favourite that you can get her to eat? Also, has she started crawling? The weight really drops off them when they start moving around.
When you say "milk" - if it is bm or baby formula, then your baby should be fine as bm/formula will ensure that she has enough nutrients. Remember, babies have extra stores of fat for just such times. The reason why it is good for babies to be a bit "chubby" is so that when they are a bit off colour they can use up their stores of body fat without it being a problem.
Let us know how things go!