Breastfeeding & Spotting & Abdomen lump - Implantation?Breakthrough? or a cause for worry?

I've been breastfeeding exclusively for 14 months and I'm still breastfeeding now. To date, I've still not had my period and my last menstral period was March 2005.

2 weeks ago, I noticed my discharge was egg white like (so I could be ovulating) for 2 days and I had intercourse with my husband around that period, Few days ago, I had heavy brown spots (I needed to change panty liners 3 times a day) for 4 days. Now, my vaginal discharge is back to the normal clear discharge. Together with the spotting, I had water retention and blotedness(still having these symptoms now) and bad headache together with extreme tiredness. I had thought I was pregnant again but when I took the test this morning, the result was negative.

Also, recently, I can occasionally feel a lump around my left abdomen near (or maybe in) the uterus which moves if I press my hand to it. It does not cause any pain but I do occasionally feel some light cramping at the lower abdomen region (pre-menstral cramps?).

Because I've not had my menses for so long, I'm not sure when is the right time to test for pregnancy. However, I read that implantation bleeding should not last for so many days so I've almost ruled out pregnancy. I'm rather confused with whats happening and I'm not sure if this is any cause for worry. Should I contact my gyn for a checkup?

Please advise. Thanks.