I cannot believe this was not already an answer.
Speak to an employment attorney.
I am not a lawyer, but you have clearly been discriminated against for your pregnancy. If your pay was lowered, you also have demonstrable damages. Based on what you've said in your post, a lawyer can probably point out a whole lot more labor law violations too.
Do not speak to HR. If you intend to sue your company (you probably should, but again a lawyer can advise you better), their job is to protect the company from you.
You may be afraid you're burning bridges. You're not; you're manager cannot be relied upon to give you a good recomendation anyway (unless a court forces him to), and since you're competant, you can find a job with much better working conditions.