Why am I allergic to calendula now?

So I'm not someone with lots of allergies, so when midway through summer last year I started having trouble with calendula it was disappointing and unexpected.  We'd been using it in salads (leaves and flowers) for a while, with at least one other green in each salad, we weren't eating it every day.  So when it got to be where my throat felt tight and dry whenever I ate it I was perplexed.  Like it never was unsafe, just kind of uncomfortable.  So I kept eating it for a couple of months since we had plenty of it, but eventually I was like okay this isn't a good idea.
It being the self-seeder that it is we have plenty again this year without me having planted it.  Though its pretty, probably pleases pollinators and I harvest the seeds to seed save and sell, I feel frustrated that I don't get to eat it this year.

Another weird thing, I tried making it into tea last year to see if that was better, the flowers I mean, and that worked.  So I guess I can make tea with it, but I'm really bummed that we can't eat the leaves and raw flowers in our salads anymore.

So why?  What causes someone to be allergic to something?  I know sometimes it can go away eventually but often it doesn't.

I'm sorry you're experiencing that - especially with such a lovely herb. Calendula is an allergen for a lot of people - many plants in the Asteraceae family are. And, it's not at all uncommon to develop an allergy or a sensitivity over time, especially if there's an 'over-exposure'. Also, even our bodies' immune systems change over time. For example, with each pregnancy, I developed a new allergy. The pattern was noticeable, by my third one, but thinking it was all in my head I asked my doctor about it, and he said it was actually not at all uncommon. Our immune systems go into sort of a 'hyper-vigilant mode',  during pregnancy, and sometimes, they don't mellow back out, afterward. Other hormonal changes, exposures, stress levels, and environmental changes can increase or even cause sensitivity to pretty much any stimulus.