What spider was it? Did you find it or can you identify it in a picture?
Do try & apply some ice on the bite lesion unless you already have.
Apply a topical hydrocortisone cream
preferably one with an inbuilt antibiotic like fusidic acid
Take an antihistamine like fexofenadine
180mg once daily
Dont worry about the leg twitching.
If your stomach continues to feel weird, then visit the ER & also do get your vitals eg BP, pulse checked..if all well, relax..you are not likely to become the next spiderman ;-) (just kidding)
Do hope all well..Wish you continued good health!
Dr Praveen Rodrigues MD