It could be any number of things. You should visit a rabbit-savvy vet to find out for sure and to get treatment, if necessary. If you don't have a rabbit-savvy vet, there are many resources online that can help you (refer to the links below, for example).
Some things it could be include:
There are many possible causes. Cyst, benign tumor... There are lymph nodes in the area and you should have a doctor examine the lump.
It could be a hernia...definitely have it checked out by your veterinarian.
it feel like aknot
maybe appendicitis
No, the male rabbit doesn't have the scrotum on top of his penis. Male rabbits have two scrotums located below the penis (closer to the anus), one right beside the other. In babies, the testes are in the rabbit's abdomen: they descend into the scrotum some time between 9-20 weeks. Adult male rabbits, when they're terrified, can also pull their testes back into the abdomen. A male rabbit who's been neutered won't have any scrotum because they'll have withered away. If your male rabbit has a lump above his penis, something is wrong. It could be a tumour, or maybe a parasite, or something else -- it's hard to say. You should bring the rabbit to the vet for treatment. See the related questions below for details.
Male cats are not spayed through the abdomen. They are castrated, which is done by cutting off their testicles.
If your rabbit has a big lump on its face, it requires immediate veterinary care from a vet who has experience and knowledge in rabbits. See the related question below for details and links.
It may be a hernia. See a doctor.
If your rabbit has a lump on its back, bring him to your usual "rabbit-savvy" vet as soon as possible. In the mean time, pay very close attention to your rabbit and look for other signs of ill-health -- if your rabbit is really sick, you might need to bring him to an emergency vet's. Even if the rabbit seems healthy, though, you need to find out what the lump is, so your vet will probably take a cell sample with a needle and order a cytology. The lump could be a tumour (although this isn't especially likely), or an abscess, or a cyst. If it's an abscess, you'll have to treat the infection. If it's a cyst, it might just go away on its own, but if it's causing problems (impeding mobility, or growing really big), you might need to remove it with surgery. See the related questions below for help.
a clump or a lump? a lump could be an ingrown hair
You didn't mention where the lump was. However, ANY LUMP should be checked out by your doctor. See your doctor as soon as possible. It may have nothing to do with the lump and you could have just picked up a virus or it could be a medication you are on.
If the pain is above your Pelvic bone to the right of your bladder, it could be appendicitis. You can tell easiest if you press on your abdomen there and release it quickly. If the pain is worse when you do this, likely as not appendicitis. Go to the emergency room or clinic.