Hmm.....I think you need to discuss this with someone who knows what they're talking about and who can see you feed, red.
I deffo wouldn't assume your 'diagnosis' of dysfunctional let down is the right one.
There are other, frankly more likely, explanations for your dd's behaviour, for example, a baby who is off-colour and fighting a virus.
Other possibility - yes, reduced supply, but only if you have over a few days reduced your frequency of feeds.
Yet another possibility - sore mouth on baby, as a result of thrush, possibly.
Don't know what the GP expects the lab to find with a sample of your milk - culturing a sample for thrush is very unreliable, and they are almost certainly going to find bacteria in it, which would 'confirm' mastitis, but actually only reflect the fact you have normal bugs on your skin.
Whatever - you need to see someone, I think.