Cluster Feeding: Honey, this is soooo normal! I wish I could tell you differently, but it is. It's also VERY possible she's going through her 3 week growth spurt a little early. They typically have growth spurts at 3, 6, and 9 weeks, and 3, 6 and 9 months, and these are periods of time where they cluster feed, and even nurse when it seems there is nothing there! But they do this to help up your supply to meet their demands. It's a trying time, yes, but necessary. I LOVE kellymom.com for the evidence based information she provides on breastfeeding. This is an article about growth spurts http://www.kellymom.com/bf/normal/growth-spurt.html and this is one about cluster feeding http://www.kellymom.com/babyconcerns/fussy-evening.html so you can understand the normal course of things!
As for the overactive letdown, I am with you. My son "powered through" them, but my daughter would pop off, become upset, and not want to nurse (she's still nursing, though at 16 months). The best and most time-efficient method I found was having a cloth nearby (cloth diaper/prefold or burp cloth) and when that crazy let down begins, I would clamp the cloth over and absorb a little of it, then latch her back on when it was calmer. I hated the thought of losing that milk and would have loved for it to be stored, but you're right - who has time for the pump? I did manage to pump it sometimes too, but that's because with my second I bought a manual pump, the Avent Isis (HOnestly, it worked as well and gave me just as much output as my Medela PIS, just one breast at a time!), and it made it much easier to pump off a little.
If your letdown is REALLY over active, you might need to do what I had to do with my daughter. It's called "block nursing". Basically, instead of feeding on both sides at one feeding (like my son required) or feeding off the left, then feeding off the right next time, I fed off the left, then when she nursed again, fed off the left again. However many times they nurse in that 2-3 hour block, you do it on that one side, then switch for the next block. This should probably be used only if you are sure that oversupply is a problem, because it WILL adjust your supply (down). Block feeding really helped with my letdown, and helped my baby. She nursed for 8 months exclusively (absolutely no solids nor formula, nothing but BM), and slowly started solids....not even really eating them until almost 12 months old. NOw she's 16 months and still nursing. So it helped control that oversupply I had and led to a wonderful nursing relationship. THis is an excellent page of good ideas for having a forceful letdown, and it explains (better than I did!) about block feeding: http://www.kellymom.com/bf/supply/fast-letdown.html
Eventually, your body will become accustomed to her demands, and you won't have that overactive letdown as much.
I hope all this helps, and was not overwhelming! With my son, I went back to work when he was 3.5 months old, but he nursed until 2, right before his sister was born, having nothing but BM for the first 7 months of his life, and never had a drop of formula. I feel like I dealt with sooo many things, from cluster feeding (esp. after I went back to work) to the struggle to get him to take a bottle (not until the 2nd day after I went back to work would he take it from my mom), to night waking again in order to get the calories he was missing during the day (he hated the bottle and would only take about 8 oz while I was gone, and made up for it in the afternoon, evening, and over night!). Feel free to message me if you have any other questions! Kellymom.com is an excellent source, though, and one I generally quote (can you tell I wnat to be a lactation consultant one day?!?)
Good luck!