Dianne:
Please don't worry: it's typical for Fifth Disease to quiet down and then appear reddish and netlike all over the body. This can last for weeks before it fades, and it doesn't mean a thing.
You also may be overreacting a little when it comes to the spider bites. When you go to an Emergency Room, the doctors, who often don't know a whole lot about dermatology, tend to call every red swelling an "infection" and treat it with a shot. Of course, it gets better, but very likely it would have anyway, because spider bites rarely if ever get truly infected. Myadvice therefore is the next time your daughter gets a "reaction" like these, show it to your pediatrician or a dermatologist if you can. You may find that thngs are more OK than you feared.
Best.
Dr. R