I have a throbbing pain in my lats after doing pull ups at the gym. I am in great shape, but can't do pull ups anymore as the lat pain is so severe.?

Go to the doctor. Difficult to tell without examining; please go to the doctor and get examined...Sounds like you tore something; and i would advise resting the muscles until you get examined.
Lat dorsi spasm. Trigger points in the lat dorsi cause pain in the mid back below the shoulder blade and can refer pain to the lower back and stomach. Lat dorsi involuntary muscle twitch has been reported to occur with severe local pain. Rest, therapy aimed at stretching, ice and/or heat and lastly blocking of either thoracodorsal nerve or brachial plexus by resection of the nerve may be indicated. See your doc.