Oh, I feel your angst!! Your baby (and mine) are probably perfectly normal. What's important is if your baby happy and gaining weight? She's perfectly healthy then. Breastfeeding is so stressful, isn't it? I'm constantly worrying and wondering if my son is getting enough, then I weigh him and he's gained another ton.
My 10 week old son has been doing the exact same thing for about the last month. I call him the volcano, because he erupts so unpredictably. It's always when I breastfeed, never when I give him expressed milk in a bottle. I thought maybe he was nursing too quickly, but he sucks down a bottle like there's no tomorrow :-)
What is happening is that the sphincter that closes between her esophagus and stomach is still immature and doesn't close completely, so that the milk just comes back up. This is perfectly normal. What you are seeing, if it's still liquidy, is a mixture of milk, mucous, and saliva. It looks like a lot, but try spilling a teaspoon or tablespoon of water on the counter and that will look like a lot too.
It's happening at this age instead of from the start because she's now starting to take in larger quantities of milk, and also your breasts are starting to up their production (are your breasts squirting milk out? I can pump out an additional 5-8 oz immediately after nursing him - I have to because it hurts!). Also, regurgitated/partly digested milk (the curdled stuff) can also still "erupt" up later, again because that little sphincter hasn't developed yet to fully do its job.
Something to consider is if she in pain? Is the spit-up really projectile? Then you may want to consult with your pediatrician about reflux. If your baby is happy and gaining weight, then she's probably just fine.
My son usually smiles and laughs after spitting up; ok sometimes he gets cranky too but isn't in pain. I've changed so many clothes (his and mine) and burp clothes, it's crazy! He can spit up just as I'm raising him upright to burp; he can spit up during and after burping; as I'm turning him around to switch breasts; always when I'm changing his diaper; and 2 hours after a nap. Interestingly, he never spits up during tummy time. And of course, he spit up all over me the first time I wore nice "grown-up" clothes last week.
Yes, he could be overeating, but there's no way to really quantify how much he's eating at the breast unless you weigh him before and after nursing. From a bottle, my son takes in between 2.5 and 4.25 oz. Who knows what he's doing at the breast except from what I express afterwards.
I've gotten advice to feed him upright, to take him off every few minutes and burp him, and to feed him less but more often from my lactation support group. I, personally, am not into the feeding less and more often, because I want him to "learn" to take in more but less frequently so that he can sleep longer thru the night without getting hungry (we're up to 7 hours of sleep at night). Also, nursing every 2-2.5 hours is just crazy. Some may disagree with my methods, but at least we're all happy sleeping at night. My pediatrician has given the ok to let him sleep as long as he wants thru the night now.
I've found nursing him with his head slightly higher than his body helps. One mom in my support group nurses with her baby in the Baby Bjorn. If you are sitting, leaning back a little so that you're not hunched over your baby helps too. Also, burping him more frequently (sometimes I feel I'm burping him longer than actually nursing him). Also, trying to stay relaxed with a peaceful state of mind helps a little.
The moms and lactation consultatnts in my support group assures me it gets better. My pediatrician reassures me it gets better. My mom reassures me it gets better. My in-laws....well, my in-laws tell me my husband spit-up until he was four. I'm hoping our son doesn't take after him. Oy. That's a lot of laundry!