The first week or so can be very crappy, it does get better. The headache is usually due to dehydration and lack of calories. I agree totally with LMW, concentrate on getting at least 2 litres of fluids in a day. I know it's a lot but just keep sipping away all day. Some people cope with cold fluids but I found my Sleeve liked peppermint tea the best. If plain water doesn't sit well add a little low cal cordial. The anaesthetic can also make you feel quite "hung over" for a few weeks. Later on Constipation can cause headaches and nausea.
I couldn't cope with soluble panadol either, made me heave like there was no tomorrow - (serves you right, nurse who made me take it, even though I told you I felt sick - she had to change the bed, my gown and my dressings in the middle of the night!)
Headache is not a sign of post-op blood clot (DVT, PE) Signs of blood clots post op are red, swollen, painful calf, shortness of breath, chest pain, coughing up blood. Hopefully you have none of these symptoms.
Hopefully you will start to feel better soon.