Flu shot and persistent low grade fever

Hello,
This is worrying me a little: I'm 29 years old and I got the flu shot last thursday (27 Oct) like I have been doing each year. But I started having some low grade fever this weekend: my usual body temperature is between 35.5-36°C and this weekend I had 36.5°C. I could feel it because I had some mild muscle aches in my legs.
Tonight, 01 Nov, I had 37.3°C and had some lower back pain. And, worrysome like I am, began panicking about the Guillan Barret Syndrome... could this be it? I don't feel sick. I just got some mild pain and this fever.
A few things to keep in mind:
- I have been excercising lately so those leg pains of this weekend could be some muscle soreness, but still... my temperature was higher than usual
- I had to take antibiotics for 10 days because of an infection to an injury I caught. I took the last antibiotic pill the day before my shot
- I can't remember having gotten any fever the other years when I got the shot
- Low grade fever that lasts this long can't be because of the shot any more I suppose?
- I've had lower back pain in the past because I'm tall and I sit at a PC all day at work, but this came up pretty abruptly

Also, what's strange with me is that it has happened to me a couple of times that I got a fever and felt really bad (feeling feverish with muscle pains and all). I measure my temperature and I get 36.6°C. Then, after I'm cured, it has happened that I measure my temperature just out of curiosity and realize I actually have 36.5° but feeling perfectly fine. And I didn't even measure it after an intense excercice or after eating something hot or anything. I always measure my temperature in my mouth, under my tongue.
Anyone can explain this?

I'm not gonna lie: I'm worried. I dont like having unexplained fever for so long. It starts making me think about all kinds of disaster scenarios and even leukemia has crossed my mind (probably ridiculous I know but eh)
I'm planning to visit a doctor but I was hoping I could at least get some advice here too.

[Dr. Kokil Mathur]

Hi! Since you have a temperature regulation problem, the higher body temperature may not be due to flu shot. It could be due to over-exercising causing dehydration and electrolyte imbalance. Persistence of infection for which you took antibiotics is also possible. It could be due to flu shot vaccine if all other causes, including flu infection that entered body before immunization or a new flu strain, is ruled out.
Do discuss this with your doctor and get yourself examined. Take care!