What is the difference between the following two sentences?
- I have had a headache since this morning.
- I am having a headache since this morning.
I've had a headache since
*I'm having a headache since
As Barrie points out, today morning is not grammatical in British or American English;
Neither is grammatical in British Standard English, where what you want is I've had a headache since this morning. However, I believe today morning is found in Indian English.