Here is my guess. I bet that your daughter has become a more efficient nurser and is able to drain the milk quicker.
All babies grow at different rates. At 3.5 months, they don't gain an ounce a day any more. The growth rate dramatically tapers off from here on out. So that daily weighing is probably making you a nervous wreak! As adults, we fluctuate around five pounds per day. Babies will fluctuate, too. She will go up and down with poops and pees. 1/2 cup of water weighs 4 oz. That is not a lot of water!
Also, breasts respond daily to demand/supply. You have had to work up a milk supply due to some early intervention that gave you a set back. I think it is hard psychologically to get over. I actually quit breast feeding and then relactated. But, as someone who relactated, know that my body stopped making milk altogether. I started demanding and my breast started supplying. I worked up from 1 drop of milk to a full milk supply. The reason I am saying all this is that breasts are responsive.
If you are not putting baby on a schedule, but nursing often switching back and forth on both breasts you will have the amount of milk that is right for you and baby.
You said that you are not sure if baby is popping off because of being full or because of low milk. Does baby stay on if you squeeze/compress your breast while she nurses? Compression increases the flow of milk. If she pops off when you compress, and acts satisfied and happy, then you know that she is full and didn't want more. After you have nursed on both sides, don't think that the first side is still empty -- it is not! Put her back on the first side. She will get another let down. Use compression again. Go back to the other breast again if she will take it. When she is full an uninterested in the breasts, stop an enjoy play time!
I think that my baby started staying awake more at about that age. He did not always fall asleep with each feeding anymore. Prior to that, I would know he was full because of the milk induced stupor. Could that be some of it? Is she not going to sleep with every feeding?
Anyway, rest assured that milk supply is not set in stone. It goes up and down to meet demand. Having a low milk supply is a temporary situation. Nurse as often as she will nurse and rest assured that your supply adjusts.
Also, it will not hurt your baby at all if in fact you do not at this moment have a full supply and she does not grow for a few weeks. Human bodies are amazingly sturdy. We are able to endure a lot. If a baby gets sick, they often will loose weight -- sometimes a lot of weight-- but when they get better, they have a growth spurt and catch right up. I am not saying that you should ignore weight by anymeans, but don't be fearful that in your quest to exclusively nurse you will in anyway damage your precious daughter. If you do not have a full milk supply right now, she will nurse you up to one if you do not supplement.
If after a couple of weeks, you don't see any change in wieght, or a drop in weight, you will know that she still needs a little supplement. There is nothing wrong with this, either. You have been supplementing in order to breastfeed. That is the right reason to give formula! (Versus to give formula without trying to breastfeed). Give it in an SNS and be proud of yourself for all the wonderful breastmilk that she is getting!
You sound like a really dedicated lady who loves her baby girl enough to hang in there What a woman!