From what I've read, hot peppers get hotter with maturity,sweet peppers mellow with maturity. I haven't done this myself so I don't know first hand.

The only thing I can think of is that the seeds of the sweet were somehow crossed with a hot the year before. Cross-pollination occurs easily between hot and sweet peppers, and the hot generally wins out. Did you start your own seeds, or buy plants from a retailer?
Also, you might want to lower your PH some. The higher it is, the harder it is for plants to uptake nutrients.
HTH,
Susan

Thanks so much for the info! I bought plants from a feedstore, then planted seeds I bought. They were different varieties of sweet bell peppers. I waited on all of them to turn red before harvesting. After weather forecast of a hard freeze, I picked them all and started the process of canning. When I was slicing them, my hands started burning, and it just got worse from there! I ended up throwing them all away. Before I plant any this year, I'll amend the soil - I'll probably just grow them in pots.

Thanks so much for the help!

That is odd. I would buy your peppers from a different place . I grew a hole slew of different peppers , hot and mild, all mixed way to close together. and they all were either hot or mild as they claimed. I do not use my seeds because of the cross breeding.