Hello and welcome 
I am not a neuro, but from what you’ve told us, I think it would be safe to say that your MRI scan probably showed a few small lesions that were consistent with inflammation, but not typical of MS, and so, as you have only had the one attack (the ON), you would be put into the “CIS; clinically isolated syndrome” diagnostic category. [There is also the possibility that you did not have a CIS at all, but instead had something called ADEM, a one-off attack very like an MS attack, but instead caused by a virus. ADEM does not happen more than once unless someone is unbelievably unlucky.]
The way MS is diagnosed is called the McDonald criteria. (If you google these, you’ll be able to find out more, but make sure and read about the 2010 revised version. You can download the whole paper if you look for “Polman et al. Diagnostic criteria for multiple sclerosis: 2010 revisions to the McDonald criteria. ANN NEUROL 2011; 69: 292-302.”)
The McDonald criteria stipulate that, to be diagnosed with MS, you need to demonstrate dissemination in time and dissemination in space. Dissemination in time means having at least two attacks or a 12 month period of gradually progressing symptoms (often involving walking, but not always). Dissemination in space means having at least two areas of the central nervous system affected. Someone who meets both of these criteria, and in whom all other MS-mimics have been ruled out, will be diagnosed with MS. Someone who meets only one of these criteria is diagnosed with probable MS. Someone who partially meets both criteria (one attack, one part of the nervous system) is diagnosed with CIS.
The fact that your foot is warm and swollen means that MS is a very unlikely cause. I think it is much more likely to be an injury so it would be worthwhile seeing a sports physio or good sports oesteopath. [If MS was affecting your foot, it would be much more likely to be painful, but be completely normal to look at and feel normal to the touch.]
So where does that leave you? On average, someone who presents with ON as a CIS has a 50% chance of never getting MS. Having visible lesions on MRI slightly increases the chances of getting MS in the future, but not to 100% - plenty of people never have another attack and never get MS. (The fact that your lesions were not typical of MS may reduce the risk too - and increase the chance that it was actually ADEM and not CIS at all, in which case you won’t have another attack.)
All in all, I strongly recommend that you try and put the whole thing behind you and forget it ever happened. There is no point in constantly worrying about something that might never happen after all! Go and get on with your life, don’t look back, and enjoy!
Karen x