Damp behind skirting boards

[AJG]

Good evening, i could do with help please. Quick profile ... 2 bed inner terrace built 1901, kitchen and bathroom extension put on sometime in the mid 60's, i bought the house 1990, 3 years ago i had new double glazed windows throughout and a new back door (this is in kitchen next to lounge window) today i decided to take out old skirting boards under living room window and discovered that they were mouldy behind and the wall in the recess under the boards was wet, i scraped out and hoovered up all of the loose rubbish and i have so far sprayed the wall and floor with a mould and mildew spray. What can be causing this .....
Could it be
New lounge window not sealed properley and rain water getting in and settling?
Could the damp proof not be working anymore? Although i don't think any dampproofing would work as the house wasn't built on solid foundations (as many in 1900's weren't)
Can anyone throw some light on this for me, i don't have any spare money as i'm on short time at work at the moment so a cheap solution would be very welcome
Thanks
Alison