Hi, as suggested by Dr Gan below there are many reasons for your symptoms, but the likely cause in your situation is either related to your recent change in diet (drastic drop in calories even if intentional) and/or the onset of menstruation (if heavy blood loss)
Occasionally, there are uncommon types of "dizziness" which can be caused by posture as your internal "thermostat" for blood pressure maybe a bit more sensitive . This causes postural hypotension (i.e a fancy name for "low BP when you stand up" ) but this may need formal testing with tilt-table test which can be done either by general physicians or cardiologists.
If on returning to a reasonable diet (i.e. please seek help of a professional dietician or family physician who knows you well) and once the periods have settled if you continue to have these symptoms please do seek medical help.
In the interim do take care especially on arising from a sitting or lying posture - do get up slowly and with support and give yourself a minute or two before movement. drink plenty of water especially on a hot humid day.
But it is likely that a visit to your family physician will sort this out for you and a cardiologist visit is less likely to be required.
Wishing you good health!
