I Have Heart Pains?

Hiya guys, I've just found this forum after doing a search for my problem. Today I went on a walk with a few family & a friend. We ended up walking around 9 miles. I am not that used to excercise, I only do bits & the longest I walk I do usually is not even a mile. I struggle to do a lot because I have asthma which can get pretty bad but I'm trying to do more. After the walk I was fine, was just aching a bit but that's totally gone now, I did have to take my inhaler a few times while walking but overall my breathing was ok. Around an hour after we got back, so it's been going on for around 6 hours now, I started to get chest pains (which feel kinda dull) and can feel my heart sort of beating heavily & fluttering. I should mention I do have a heart problem (just a small hole in my heart, not sure of the proper term) but I've had this for years, it was found by accident & has never bothered me. The pain calms down when I've rested for a while but I can still feel it & seems to get a lot worse when I move (I only have to walk a few steps & it absolutely kills me) it doesn't feel like the chest pains I get when I'm having an asthma attack. Does anyone have any idea what this could be & what's best for me to do? Thank you so much, hope someone can help me!