With my DD I seemed to have very specific low supply in the evenings from around 7 to 9 weeks. Looking back, it was during the days when I ran around after a 2 yr old as well as tending to a newborn's needs. I tried my best to up my supply through pumping and in the early days, even fed her with a bottle to get her to settle. I actually pumped around the clock during the 6 wk spurt (it was at 7 weeks in her case and I really thought I'd escaped without it till then) and after the 7pm bedtime feed, I would try and settle and she simply would not without 1-2oz of EBM in the bottle. This made me realise it was low supply for sure....
in our case, it was due to OS in the morning, easy and OAL then, followed by stress, tiredness, exhaustion as the day wore on and less and less active letdown. My DD turned out to be a very fussy, impatient feeder due to tongue tie (which was not yet diagnosed) but she did very well despite the tongue tie due to a very good milk supply in the night and till around 4pm in my case.....
By around 9-10 weeks, I was getting more rest, DD was doing better at the breast and my supply had built up to being OS and OA a lot of the time, even in the evening...and we were doing better.
First and foremost, I advise you to relax and get rest - it worked wonders for me and my supply. But I definately went through what you are going through now around 3.5 yrs ago xxx