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.
I remember reading something about either MLB or NFL players training up in the mountains to boost their red blood count, and then getting transfused before key games.
Only way to catch that is blood test.
Do you still swim for fitness? I've observed that often, competitive swimmers get so burnt out and so obsessed with making speed their top priority, that they have a difficult time enjoying swimming as a fitness regimen later in life.