I was having some severe cramping went to the e.r they did a serum blood draw and my hcg was 18,265 they then did a vaginal ultrasound and said I was 5 weeks and 1 day,do I need to be concerned with twins since it was not a molar or an ectopic

Twins could be a possible reason for the higher than average level. Could the dates be off a bit? That often plays a role as well since there is a wide range in levels that is considered normal. They may do another check on your level and will likely do another ultrasound within a few weeks if they feel the need to check for a possible twin pregnancy.