Very faint line, now tests are negative.

So I got an extremely faint line this am and was sooooooo excited! So I went out and bought a first response test this afternoon to double check it, and it's come back as a BFN. I'm still 3 days before AF due. But surely if an Internet cheapy picked up something then FIRST response would? To say I'm absolutely gutted is an understatement!! We BD every day of my fertility window. It's our 1st month of really trying to conceive as in monitoring everything. But we have been without contraception for three months nearly 4 now. Has anyone else been where I am? I feel so down and lost now. I'm meant to be going out tonight, but all I want to do is get in my pjs, eat, and sit on the sofa in self pity!