Can you check your company policy in more detail? I agree that your levels seem high, but I'm wondering if your manager is following the standard policy.
Ours is that people have to have a back to work interview after any uncertified sick leave from the third instance in a 12 month period onwards (managers have discretion to do it from the first instance if they want).
If all unrelated (i.e. just general bad health) it's put (tactfully) to the employee that they need to manage their health more carefully as they can't constantly be missing work that they are paid for.
If it comes out that there is an underlying condition then the company needs to know in case there are ramifications/alterations required (i.e. back problems, provide proper seating, diabetic, provide facilities for injecting if need be. Safety for the employee and others needs to be considered). The employee does not have to disclose to their line manager but needs to talk to HR in any case.
We have a company doctor that we can refer employees to, if their sick leave is too high. I think your manager should really be getting HR involved rather than sending you back to your doctor - if your leave was all certified, the doctor can't really say anything else that will be helpful.
Just to point out that a LOT of people believe that if sick leave is genuine then the employer has no grounds for action - i.e. if the person can't help it. However, if a person has agreed to a contract of employment, then they have to stick to it. Most contracts allow for reasonable leave. Excessive sick leave, even if it's genuine, means they are breaking their contract and causing the company losses. If companies had to hold on to all employees like this, they'd go out of business - or else only offer fixed contracts to all, and would not renew for anyone with a poor sick leave record.