Sudden Headaches

I have been lifting weights off an on for about 2 years now.  Suddenly I am starting to get really bad headaches once my heart rate gets elevated.  The headache starts at the bottom, back of the neck and goes up my head and finally towards the front and all over my head.  They last for some time--normally if I lay down and rest it goes away.  I don't have a history of headaches and rarely get them.  Any advice?  I have heard possibly it could be--strain, high-blood pressure, brain tumor ( i hope not) or exercise headaches.  Any help would be appreciated.