the answer is no to taking the extra pill. diarrhoea doesn't really affect pill. if it's the combined pill you can take it up to 24 hours late (i.e. two pills together the morning after your missed pill) and you're still protected. if it's the minipill (assuming it's not desogestrel/cerazette/cerelle) then within 3 hours means you're fine or within 12 hours if it's desogestrel etc.