Why is ingesting blood bad?

i get chronic nosebleeds (i've had them cauterized, but its wearing off), and occasionally i look up at something, and blood runs down my throat, ive heard that this is bad, ever since my nosebleed started, many years ago. After my nosebleed, often 12-24 hours after, i get a slight stomach ache, if i'd ingested blood, why is it bad for you? and how can it cause stomach aches??