I am getting over a cold. when i blow my nose my left ear pops and i get sudden vertigo. what should i do?

NOTHING or OTC med: You probably have 'eustacian tube dysfunction.' this happens when you have inflammation of nose and throat from a viral upper respiratory infection, 'a cold'. The swelling blocks entrance to 'the tube that runs from your throat to inner ear' .The inner ear controls balance hence vertigo is associated. Decongestatnts may help or just waiting.

Eustachian tube: When you are blowing your nose you are forcing air into the eustachian tube which connects your nasal passages to your middle ear. Antihistamines (zyrtec, allegra, benadryl, (diphenhydramine) etc) can help keep that tube open, but often this takes time for the inflammation to settle down. If it is really bothering you, see your doctor and see if a steroid nose spray would be beneficial.

Stop blowing : Not a good idea to blow your nose if you are getting vertigo (the sensation of motion). It could mean a small potential leak or irritation involving the connection between the middle ear and the inner ear. Wipe or sniff but hold off on blowing. If the vertigo lasts get to an ENT doctor right away.