Hard to say Cecilia. I'd take notes on her behaviour, see if there are patterns emerging and get her to your GP for a check up.
She may be a 'happy chucker' who spills a lot and gets uncomfortable because of the mechanics of spilling ie. she might get a wind bubble traps which hurts and she spills as the burp comes up or dislike the sensation. My newborn seems to be doing that at the moment, gets very squirmy and uncomfortable but seems OK after a burp and spill other than being a little upset at the taste/feeling. If she is nodding back off to sleep after the spill and isn't continuing to be unhappy then it might just be that. Fingers crossed.
Kids with GORD (who spill and have excess acid production) will usually be extremely unsettled (ie. scream all day & often all night!), often be fussy feeders (scratching/biting/pulling at the nipple/bobbing back and forth with hunger at the breast wanting to feed but being reticent to), dislike being put down, very difficult to get to sleep ever, frequent wakers etc. and its extremely distressing and hard to cope with.
It can take some babies a while to become fully refluxy, so it may be that her symptoms develop further in the next few weeks into the unsettled behaviour, but I'd get her to the doctors anyway about the spilling (though if her weight gain is good then that's a good sign). If they don't develope you might want to try a feed thickener like the one Karicare make which is a maize based product which can be fed on a spoon after BFing which might reduce the amount of spilling.
Also have a look at
http://cryingoverspiltmilk.co.nz for good advice.
Good luck, let us know how you get on...