Implantation failure, sore lower back

Hi
My husband and I have been TTC since January, when my Mirena iud was removed (it was my second Mirena, in total had Mirena about 6 years and loved it). We're both aged 30 and healthy.

Its now May and i know we haven't been trying for long in the scheme of things. But the same pattern keeps happening to me and I was wondering if anyone else has experienced this? I feel so deflated and stressed each time.

Three times in a row now I get light brown implantation bleeding about a week out from when my period is due. Seems promising but then either on the third day of this, or within several days of it stopping, suddenly it'll turn to red blood and effectively I get my period with horrible cramping.

The symptoms during this time are different to when I have my regular cycle and period. In particular, the first night before the brown spotting starts I can't sleep. Usually I wake up in the middle of the night with lower back pain. This lower back discomfort continues for the next few days with the brown spotting (I find it hard to lie on my back or press my spine against a chair). Sometimes it feels like it burns at the base of my spine.

Anyone else been through this? I am wanting to know what is wrong with me so I don't have to keep going through this pain. Maybe I need to gets some tests done at my GP? Any advice?