Suddenly i just dont have strength to exercise!?

I have been religiously eating healthy for past five months and lost 34kg aswell. First i wast on a low carb diet but now since last week i am eating normal food but in moderation. I used to work out around 90 minutes a day which included 70 minutes jogging, 150 situps, 80 pushups and 100 bicep curls. But suddenly now i just dont have the strength to exercise, i run for 20 minutes and get headache, abnormal heartbeat and get out of breath. I am so stressed that i will get fat again because i m not able to exercise. Also, i havent eaten any junk food or fast food since 5 months now because i just dont like these two for some reason. I also try to avoid white rice but eat it occasionally. I eat complex carbs like wholemeal, wholewheat and fruits. Please help me and just to make it clear i am sleeping well. I am still abit overweight. Also, i get bad headaches when i wake and get out of bed or when i standup after sitting on a sofa for hours. I was previously drinking 4 litres of water a day but now i am down to 2.5 litres. I am always feeling weak and i find it hard to understand things. Please help me ! :(.