yes, it's really highly unlikely that your milk would have just gone. expressing is NO indication of how much you are making, your baby is far more efficient at milking the breast. some mums can never express anythiung, but still breastfeed successfully
i think there are a few possible reasons for his behaviour:
teething- this can cause a lot of fussiness at the breast. you could try giving some painkillers and seeing if it helps
your period- has it returned? if not, do you have any symptoms of it? sometimes women find their supply drops a bit when they get their period, and increases again afterward. this normally results in baby simply wanting to feed more, but could also be a cause of frustration
other pain- does he have a temperature? pulling at his ears? sometimes when little ones have ear infections it's uncomfortable to lie in a nursing position
distractibility- sometimes they are just so nosy they simply can't get down to the business of feeding, so hungry, but can't help wanting to look around. feeding in a quiet, slightly darkened room may help
I think teething is the most likely, but I'd also advise speaking to a breastfeeding counsellor, or seeing one if you have a group locally