Guys. TTC and going slightly cray!

Kalina
First period off of BC was May 23 (42 days since my last). 
Then enters June... I'm not quite sure when I ovulated, since I was not tracking BBT, just trying to get my cycles back on track. However, my cycle apps predicted ovulation was June 15- so I'll go with it. From June 19-present (CD 41!), I've had very mild tugging-like "cramps" -off and on- for about 2 weeks. I thought they were period cramps, but they are barely "cramp-like" in nature... more like just something moving/pulling down there.
Other notables: 
-Breasts just slightly tender 
-Mild brown spotting CD37 and 38
-BFN June 22 and again on June 28.
-Some beck and back acne
Not really feeling nauseous... and I guess I've been bloated and a little tired but for me that's like every other day so hard for tell haha.
My main question is about the longevity of the cramps. I thought implantation cramping was only several days, but can it be longer? I know I should test again soon-- and will-- just looking for some reassurance from you wonderful, experienced ladies!