My hemoglobin has dropped from 133 to 120 in 6 months. during that time: stopped eating red meat, had haemorrhoids, been taking daily low dose aspirin and have been dieting excessively. have these caused it?

Yes: The short answer is yes. It is unclear whether this is a significant issue or nor. Would see your docotr.

Possibly: Anemia has many causes, but GI bleeding is probably the main treatable one, and also the most worrisome. The only person who can tell is your physician, who may have you do some tests to rule out potentially dangerous causes, like colitis, polyps, tumors, etc., or start you on appropriate treatment if it is just hemorrhoids.

Potentially: You are at risk of blood loss with the blood thinner (the aspirin) and the hemorrhoid and with cutting red meat, you are cutting iron intake which helps replete that. That is not to say that this is the only explanation for the drop. Serial monitoring of the hemoglobin and appropriate work up is warranted if it continues to drop.