Welcome to MS Central. I'm sorry that you've been diagnosed with MS, but you've come to a place with lots of information and great support. I'm sure that you will have many questions and please feel free to ask anything.
The tricky thing with multiple sclerosis is realizing that we are not immune to every other normal ailment out there. If you think that you have developed the flu, then certainly do not hesitate to consult your primary doctor. The flu this year was resistent to the flu shot so many more people have developed it.
Personaly, I have not described MS symptoms as having the flu, but if we take each example you've given, then yes, they could be MS-related. Headache is not listed as one of the more common symptoms, although it has been noted as a presenting symptom in MS. Here is information which the National MS Society has on Headache. I have friends who have described nausea and headache as being one of their worse MS symptoms.
Where I've had nerve damage related to a limb, I would describe that limb (left arm specifically) as being very aching when something MS-wise is going on.
And fatigue is one of the most common MS symptoms reported by 75% to 95% of patients. Here is a recent post which discusses strategies to combat MS fatigue.
If you've just recently undergone Solumedrol (steroids) treatment for an exacerbation, the symptoms you describe could be a result of that treatment.
I hope that this helps a little.