Pretty little volunteer, that's awesome. You may want to reconsider the soil you potted it up in... Mammillarias as a group like a well draining mix (like half rock), and don't like to stay wet for too long. You can mix regular potting soil with perlite or pumice (or lava rock or whatever you have around) and arrive at a very cactus-friendly mix.
Otherwise I would advise caution to avoid overwatering (and keep the pot under cover to avoid excess rain). The rule of thumb with these plants is to wait until the soil is dry, then water well. They are probably going to be more finicky in a pot then they would be in the ground, for what it's worth.
