It does not sound like your milk supply has decreased, but it could still be possible.
With my children, I was unable to pump very much milk. At most I would pump 2 oz out of each side, but usually 1 oz from each side. I had a very expensive Medela pump and even when I was engorged, I could only pump 2 oz from each side. I found when I was weaning my daughter that the Medela pump must not have been powerful enough because I went and bought a $30 Evenflo pump at Target and I pumped 6 oz from EACH side for three consecutive days, four times a day! That's 48 oz and that was when my daughter was 20 months old! I thought my supply had diminished by that point, but apparently it hadn't.
My children have woken up on occasion with dry diapers and I don't think that the new diaper lasted very long because it seemed that within an hour their diaper was extremely full! Has she had a wet diaper since this morning? Not sure if it has any correlation,but both of my children were night potty trained as soon as they were trained during the day-so this might be an added bonus later on:)
As for not being engorged after a long night's sleep, your breasts eventually adjust to your daughter's daily needs and you should be producing just enough for her to eat each day. The only time I got engorged after the first few months was when I was away from my children for more than 24 hours or when I was weaning and taking away one feeding per month.
Now I am not an expert, just a mom that breastfed for 36 months straight, so you might want to check with a lactation consultant. There are certain things that they can do to assess the amount of milk that your daughter is drinking, like weighing her before and after a feeding. I did this and even though it was $50 for the consultation, it was worth it! It definitely put my mind at ease:)