Definitely be careful because people follow you. Immediately after you purchase something, make sure you are not being followed and hold on to your valuables, keeping an eye on them 24/7 100% of the time.
That's how my wallet was stolen: I was being followed and immediately after I purchased somethings, the thief knew exactly where to go to steal my wallet -- this was all in a matter of 5 seconds. These people are professionals and they walk around in groups; they know exactly what they're doing.
Just in case something unfortunate like this does happen, however, I urge you to go to the police and the MRT station (if your MRT card was also stolen) to file a report as it could come in quite handy and of course call your banks right away (cell phone code to dial US is 006).