What could be going wrong? Weight training for breaststroke

I am a breaststroke swimmer, have always been. Then I stopped for 6 years and have started swimming again for around 3 months now. Around a month and a half ago I was doing 16 times 50m in splits of 42-43 at a pace of 50 seconds ea with around 70% effort. Reading all the forums I since started doing more weight training around 2-3 times a week focusing mainly on upper body as I find that my upper body is quite weak and small compared to my legs. Now, I'm finding it extremely hard to do the same splits, and usually end up going 46-47 @ 50 sec pace after around 3 or 4 and feels more like 80% effort. Same thing with my 100 splits. Was going around 1.25-1.27 and now going around 1.30-1.35. Am I causing more resistance or is there a period when the body adjusts? And shouldn't I be going much faster as I can lift more weights for a better period of time now than when I started? How can I make the weight training effectively work for me? Does anyone have similar experience and how you overcame this?For reference: I'm following the advice from this thread including others forums.usms.org/showthread.php Thanks