gas cap/lid area small gas leak?

[airhead]

91 240sx KA24DE all stock
i just bought this car not long ago

week after i bought it Fuel pump went out so i went ahead and replaced it with a new one.

right after i replaced it i went ahead and filled her up at shell gas station 89 octane

since i was low on gas (thats how my pump gave up)
at 15$ and like 7 gallons the nozzle clicked off like if it was a full tank ?????
so i was how can it get filled at 7 gallons so i kept on holding it and fuel started to leak out the gas out the filler hole. so i just guessed maybe i had some couple of gallons of gas in the tank still...

so now that my gas is low i went to go fill her up and it filled up like around 8 gallons and it did the same thing wtf???

i know i dont have a 8 gallon tank?
also another weird thing i noticed my MPG has gone down and i just opend my gas lid today and i noticed small gas leaking around the cap...

could i maybe moved something when i took out the pump?
or
i might need a new gas cap?

never had this kinda problem on my other s13

thanks
and yes i searched got nothing.

EDIT:mod please move this to tech talk section