For fundal heights, I think 2-3 weeks either side is generally fine.

Sounds like your DD has long legs... Don't worry about the differing estimates - they are all just estimates. If they're all round about 1-2 weeks either side of what they expect, then should be fine.

Possibly you have low blood pressure? Feeling faint isn't that uncommon during pregnancy. Also, if you're lying on your back you may be compressing some artery or other (can't remember name) which can leave you feeling short of breath and/or dizzy.

If you think you might be having heart palpatations, then definitely have a talk with your doc/midwife.

All the best~