As a POTS patient, you probably shouldn't ever donate blood. You need to focus on maintaining normal or even slightly higher than normal blood volume to help compensate for the POTS--that's the crux of the idea behind increasing fluid and salt intake. Were you instructed to increase your fluid and/or salt intake? The reason you had those symptoms after giving blood is because your total blood volume was slightly decreased so less blood was available to pump up to your brain; if you have blood pooling in your legs or abdominal bed (and thus, thoracic hypovolemia) which contributes to lightheadedness as is generally the case in POTS, having less than normal total blood volume (hypovolemia) exacerbates the problem. Likewise, in summer, this is where heat intolerance can also come into play. The body controls its temperature relative to excessive environmental heat by dilating peripheral blood vessels, which can increase blood pooling (ergo, worsening thoracic hypovolemia); also, in high temperatures fluid and electrolyte loss through sweat can contribute to worsening of POTS symptoms.
You may benefit from reading more about the mechanisms of POTS, Orthostatic Intolerance, and Dysautonomia in some of the articles in our further reading health page:
http://www.medhelp.org/health_pages/Neurological-Disorders/Further-Reading-on-Dysautonomia/show/696?cid=196
Is the beta blocker the only medication you are taking? On average, what are your heart rate and blood pressure when you are lying down? Sitting upright with feet on the floor (legs not crossed)? Standing (taken 2-3 minutes after rising)? (Standing values at 10 or 15 minutes may also be useful if these are appreciably different from the 2-3 minute value, as they are for some.)
Some POTS patients do not tolerate beta blockers, or may be able to tolerate a certain beta blocker but not a different one. Important information to have is what your lying/sitting/standing BPs and HRs were before you even started taking the BB, what they are at night when you feel better, and what they are during the day when you feel the BB is affecting you negatively. This can give objective information to your doctor about WHY the beta blocker is making you subjectively feel a certain way, and help guide his/her decision about what the best course of action is. Some problems can be dealt with with a dosage adjustment or a switch to a different beta blocker; in other cases, a beta blocker may not be the most appropriate treatment for you.