You know that you want to find a post, where your username and that other user both occur. (You posted an answer and he posted a comment.) So you can try to Google for both usernames. If you're lucky, there won't be too many posts. For example, if the two users were me and Arturo Magidin, I would search for "Martin Sleziak" "Arturo Magidin" site:math.stackexchange.com.
If you know about some phrase that occurred in that post, that might help you to refine the search query.

A different solution could be download all your responses, for example download all pages like http://math.stackexchange.com/users/8297/martin-sleziak?tab=responses&page=1 http://math.stackexchange.com/users/8297/martin-sleziak?tab=responses&page=100 (Of course, you have to use your the page with your username, this is just an example. You will find the tab with responses on your profile page) Or you can download the comments of that particular user, if you think you have better chance of finding that comment in those.

This can be done automatically, using Wget or some similar program.

Once you have all responses on your hard drive, you can search the files for the particular username there. (For example, in Total Commander you would use Alt+F7 and enter the username into "Find text".) I have no doubt that your favorite filemanager has a similar function.

You know that you want to find a post, where your username and that other user both occur. (You posted an answer and he posted a comment.) So you can try to Google for both usernames. If you're lucky, there won't be too many posts. For example, if the two users were me and Arturo Magidin, I would search for "Martin Sleziak" "Arturo Magidin" site:math.stackexchange.com.
If you know about some phrase that occurred in that post, that might help you to refine the search query.

A different solution could be download all your responses, for example download all pages like http://math.stackexchange.com/users/8297/martin-sleziak?tab=responses&page=1 http://math.stackexchange.com/users/8297/martin-sleziak?tab=responses&page=100 (Of course, you have to use your the page with your userID and your username, this is just an example. You will find the tab with responses on your profile page) Or you can download the comments of that particular user, if you think you have better chance of finding that comment in those.

This can be done automatically, using Wget or some similar program.

Once you have all responses on your hard drive, you can search the files for the particular username there. (For example, in Total Commander you would use Alt+F7 and enter the username into "Find text".) I have no doubt that your favorite filemanager has a similar function.