Dear Nikon camera sifus

I have a problem with my Nikon D90 and I couldn't seem to find people with similar issues on the internet and I was wondering whether you guys could help me out with this?

At Manual "M" mode, the shutter speed value always change (automatically to another value) after I snap a photo. I need to adjust to the previously set speed before I can snap another "similar-setting" photo. I'm thinking that this is not normal because its supposed to be entirely Manual... right?
The shutter speed value doesn't change at all under "S" mode though no matter how many photos I take...

Can any sifus point out what's the problem with my D90 or me?

THank you very much in advance ya!!! Appreciate it~

I'm using Phottix. No problems so far except sometimes I need to switch the batteries around because "batteries can't be recognized/not suitable"

---

Added on August 22, 2012, 2:54 pm

Contemplating whether to get SB600 or not. Many said Nissin not bad T^T

Aiya, dark condition also see how you handle it... if its fluorescent lamp lagi mafan LOL