My jeans too tight tryed on levis 501 fitted untill sat down tummy fat lodged over top waist band?

i been a waist 36 30 in my levis 501 indigo for a year or so dust bought new pair think i hadnt put on any weight try on got home were bit tight sat down tummy fat lodged over waist band cant return them took tags off wonder if stretch a bit or i might need buy next size up because seem i got fatter esimate had gone jeans size or too top button was bit hard do up tight fit wear show s im fat need go on a diet tuck shirt in show put on weight tumy overhangs eating too many crisps twix