My sugar spiked at 21.2. i had an injection for allergies earlier today. i took 40units of humilin and it came down to 17,6. up to 18. should i go er?

Not really: you want to call or see your doctor, meanwhile, continue your insulin regimen according to the sliding scale you have. The shot you received is probably a steroid shot, which can raise your blood sugar, it may stay for sometime if it is a long acting steroid. You need to go to the ER if you are not doubt, have serious symptoms of very high or low blood sugar, wish you well