my 3.5 month old dd has been fussy recently as well. at first i thought it was gas, but i take good care of that with the burping. i have realized this is somewhat common for her age range. for those moments when she just doesn't want to take it, i changed my position for her, using the side-lying on my cushy bed. she seems to settle and take it at those moments.

as far as your little one, maybe this might help? if it continues, of course, you should consult your pediatrician. hope it all works out!