Is it there was a swarm of bees or there were a swarm of bees?
"Swarm" is a collective noun and takes a singular verb. Although
many bees make a swarm, it is meant to be taken as one whole. The
correct sentence is, "There was a swarm of bees (chasing me)."
The sentence could be rearranged to read, "A swarm of bees was
chasing me."
See the related link for subject/verb agreement rules. Rules 3,
4, and 18 address this question.