mrsr
It could be a number of things.

Postural Hypotensions (where your bp is low - or drops when you go from a sitting/laying to standing position) - check with your Dr that your Blood Pressure is ok.
You might need to take the transition a little slower.

Low blood sugar - had you eaten regularly prior to this happening ... perhaps keep some barley sugar in your bag just in case

Dehydration - keep your fluids up so you dont get dehydrated and run down!

And i would give the heels a miss too! lol

HTH

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