Dirt in the bottom bracket?

[Painkiller]

you have a standard square taperd spline spindle bottom bracket so will need a crank puller and a cartridge style bottom bracket tool. you did not post where you are from.but if you have a walmart close they have what you need at the online store with free "site to store" shipping. The crank puller will be around 15 dollars and the bottom bracket tool shimano style will be around 10 bucks

both good tools to have in your stable

But what you describe could be as simple as hosing down your bike good and re-lubing everything.
sand and gunk get everywhere when you off-road. So go get a good waterproof grease also.
rubit on all surfaces , bottom bracket cups,the shell on your frame and your spindle splines
and good luck, it is an easy task and let us know how you did