I keep feeling dizzy and faint and head-ace?

i walk to the shops and it was really cold outside and hot in shop stared doing my shopping and stared felling dizzy and like i was going to faint and my version was blurred and pain at back of my head i have high blood pressure but on tablets for that i have felt faint before but not like this i was on my own with my son in his pram i felt like my body was wobbly my hands was shaking and Had to get my husband to pick me up i told doctors be four about feeling faint did blood test but come back normal can someone give me some advice thanks nikki j