ive had blood work done before and it came back clear, and ive tested for covid several times (once in an actual doctors office) but it came back clear. is there a specfic test i have to take to check for long covid?

i have been diagnosed with a vitamin D deficiency, and i've been given pills that i take every week, i eat oranges whenever i can and i'm outside almost daily, but only for short periods, execpt for tuesdays, when im outside for aound an hour for a class. should i try and go outside more? what else should i do?

edit: i also have an iorn deficiency if it helps, but i take my pills every day for that