nipple v. thumb

At some of our late night feedings when Adler is supper sleepy she constantly fights me. she will be breast feeding then take the nipple out of her mouth just to put her thumb in. I know she is hungry and wants to suck, but you would think that the one with the milky goodness would win out. I have to constantly move her hand/thumb and re-adjust and put the nipple back in her mouth. I mean this sometimes happen every 20 seconds so it is an up hill battle. I know this is why she swallows air and gets those tummy aches and loud burps/gas.