Spotting, corpus luteum defect and slow rising hcg

This is my 6th pregnancy but first baby.. I have a history of 5 miscarriages that all ended before 6th week and all started with some kind of spotting. I got pregnant this time which was an unplanned pregnancy and I didn't take any medicines until 4w5d. Which is the day I found out I was pregnant. I had all early pregnancy symptoms like nausea, sensitive Boobs, mild cramps, back pain, food aversion.. Until I started spotting on 5w2d..

Here are my hcg levels

4w6d - 1524 ( to confirm pregnancy)

5w2d - 3540 ( it's the day I started spotting and lost pregnancy symptoms)

5w4d - 4860 ( spotting continued so I was expecting a decline in numbers.. But they increased and the doubling time has reduced which is 87hours.)

Some websites say doubling time is reduced after hcg hits 1200 (72hrs-96hrs..) and some say 87hrs is lower than normal.

I went for an ultrasound on 5w2d which showed a 4mm sac and on 5w4d it showed a 7mm sac. My doctor said it's nothing to worry as the bleeding might be due to corpus luteum defect.. I should worry only if I have heavy bleeding with cramps.

But frankly I m worried coz I can see my doubling time reduced and spotting is still there.. Sometimes pink, sometimes red, sometimes brown.. I don't want this to be my 6th miscarriage . Can someone Please clear my confusion???