Maybe your symptom is the reaction of allergy caused by the grass. But, it also can't be ruled out the inflammation. It's difficult to make a judgement here, Your best bet would be to seek attention of an obgyn.
Besides, I think you should drink more water, it may do some help~
There are several posibilities for you situation. It can be the result of balanitis foreskin phimosis，anaphylaxis，sexual contact.
Advice: Stop having sex with your girl.
Wait for a few days to see if the symtoms relieve.
If don't, go see your doctor as soon as possible.
or See your doctor directly if you too much worry your situation.
Hope that helps...
There are several possibilities. It can be the result of balanitis， Foreskin phimosis ，anaphylaxis， sexual contact.
Advice: stop having sex with your girl.
Wait for a few days to see if you situation relieves
if don't, see your doctor as soon as possible
or go see your doctor directly if you fear more serious situations