It sounds like you are suffering from orthostatic hypotension
. This could be due to dehydration
(vomiting? profuse sweating?) or a side effect of a medication you may be taking. The change in heartbeat may also be due to dehydration or a side effect of a medication.
On a more serious note, these effects may be TIA's (mini-strokes) which are often caused by an underlying heart condition (such as an atrial septal defect
Either way, follow up with your physician is recommended.