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As a former competitive swimmer myself, I can tell you that beating your former best time in a 100m race by 5 seconds is a huge flag that something is wrong.

No-one makes gains like that except the people who are just starting out swimming. Once you've been training and swimming for a couple of years, you've pretty well gotten close to the edge of your nominal performance envelope. Your speed is fairly well determined by your build, muscle tone, and technique - did you get a good start, did your strokes work out well for the turn and finish (ie. no coasting or short strokes). In any one race these things add up to a tenth of a second (plus or minus) here or there. As you grow/mature and add muscle, you tend to get a little faster year-to-year.

Throughout the course of a season a good swimmer might pull their time in a 100m event down a second, maybe a little more. If they are really pumped up at a big event, or playing catch-up on the last leg of a relay (eg. the French team) they may shave a few tenths, even a whole second off a personal best.

Taking 5 seconds off is a red flag. That's a local Girl Scout troop beating a Marine recon company at arm wrestling. Something is wrong, very, very wrong.